6 mos. of working the treadmill, 4% ( I guess, not sure the marker on that) incline at just under 4 mi./hr. 5 days a week for 3 miles. Haven’t lost a pound. I eat a protein and an orange for brekkies, a “meal-in-a-box” (Lean Cuisine or other) for lunch, and a normal but reasonable dinner, being careful of carbs. Certainly some increase in vitality, and strength, but the scale tells me I am failing.
Suggestions??
Have you determined your actual calorie needs?
Realize that 3 miles is only (about) 300 calories, not really a huge amount.
Have you measured your waist and legs? A gain in muscle mass can hide a loss of fat but will show up as smaller measurements.
You clearly are eating too much. Exercise gives you an appetite. You’ll have to cut calories somewhere. Your body has ways of resisting weight loss efforts, but it’s physically impossible to maintain weight if your calorie intake is lowered sufficiently. Whatever you have done is evidently not sufficient.
But I’d be amazed if you haven’t lost fat and gained muscle after exercising like that for six months, assuming you were sedentary before then. And your heart and lungs are probably much healthier, along with all kinds of other risk factors being lowered. I think it’s been established that those kinds of things are more important than weight.
What metric makes the most sense to use?
You feel better. You feel stronger. Those seem like pretty meaningful metrics. You clearly are not “failing.”
The scale? Of somewhat more limited utility. Not none but less than the other two. Percent body fat, or, as Runner Pat suggests, some default proxy, like waist size, how your clothes fit, etc., would be more telling.
Only other comment - same incline, same speed, same duration for six months? Kick it up a notch. A few times do some faster sections ans then slow it back down, try going a bit farther in the same time period, try increasing the incline for a minute or so then back down to lower than your 4 degrees and back up, repeat. You are not failing but you are in a rut. You are fitter now than you were six months ago; push yourself a bit more accordingly.
What is your demographic (age, sex, height, weight)? How carefully have you monitored your food? People often significantly underestimate how much they really eat.
Like DSeid mentioned, you should change your workout if you’ve been doing the same thing for 6 months. Your body has long since adapted to that workout, and it won’t induce your body to change. All it’s doing now is burning a couple calories, which is a terribly inefficient way to lose weight. So increase the intensity as suggested, then do it again in another couple weeks.
Are you walking or running on the treadmill? It makes a difference if you are working up a good sweat or not in terms of intensity. You could walk 4 miles every day and still not increase the calories burned very much. And as others have noted, if you are feeling better that is not a fail. Maybe you have developed a good base and now it is time to go a little further/harder.
As someone who has lost 60 pounds, put 30 back on, lost 25, and put 10 back on, I have some experience. 
I would recommend dropping the incline and increasing the speed to at least 6mph and do that for 20 to 30 minutes. The couch to 5k program is great for helping you to be able to maintain that.
I would also change your lunch from the lean cuisine meal in a box. That processed stuff is absolute poison (okay, maybe that’s an exaggeration). Maybe make homemade soup in a crockpot or a half of a wheat bread turkey sandwich for lunch. Your body craves healthy, non-processed, natural food to function like it should.
While I like home made foods as well, I don’t see why having Lean Cuisine meals at lunch is going to be any worse for your diet than anything else. It’s not bad food, it’s portion controlled, and filling. Your body doesn’t really know the difference in this case. Exercise and meals like this helped me lose 60 lbs as well.
Focus on reducing fats in your diet, rather than carbs - fats are 50% more calorie-rich, so it’s easy to eat a lot of extra calories without knowing it.
By switching to a low saturated-fat diet, I lost 30 lbs., and my main goal was not to lose weight - it just happened.
Anecdote alert: After I switched jobs from sitting at a desk to one where I was moving all the time (combined with an effort to eat healthier) my weight dropped from 235 to 205, where I stayed for years. Then I started working out with free weights 4 times a week. After a few weeks one of my co-workers asked me how much weight I had lost, and guessed it to be 15 - 20 pounds. In reality, I had lost 2.
However, it would be more accurate to say that I gained ~15 lbs of muscle and lost ~17 lbs of fat.
Later I quit working out and then the weight came off. (ie. I lost the muscle.)
The point is, don’t rely too much on the scales. Because muscle weighs more than fat, scales are not a great way to measure fitness.
Quite the exaggeration. It’s not really more “processed” than the factory-made bread and the deli meat in your example - in fact, could even be more sodium in a turkey sammich depending on the meat. Frozen meals obviously aren’t gramma’s home cookin’ but the fact that they’re frozen removes preservatives from the table (ha!) so it’s not much different from thawing out frozen leftovers.
Concur that the OP should switch up the exercise routine and increase the intensity a bit.
Add another voice to “increase your workout intensity”. I believe that 10km (a little more than 6mi) at 10 minute miles is achievable regularly for most people (although it certainly doesn’t feel like it when you’re working up to that distance).
There’s a certain amount of energy reserves that are stored in your muscles as glycogen which are used first. If you want to start working away on the energy stored in other places (like fat) you need to get an intense/long enough workout to burn that glycogen down first.
The OP said he was doing 15-minute miles at a 4% incline. There is not much difference in calories expended between walking and running the same distance. Running does expend a few more calories due to higher leg lifts and the fact that both legs are off the ground at the same time, but not much more. “A good sweat” has little to do with calories burned. Sweat consists of water and electrolytes, and the water is quickly replenished. You don’t want to lose water weight. Water has zero calories.
From what I’ve read, once you are able to run 30 miles a week, you will note a reduction in weight due to the residual metabolism after you quit exercising. It appears that at the level of 30 miles a week, your resting metabolism is increased and you consume more calories, at least for a while after stopping the exercise.
Yah, I did not say it very well. I was not suggesting sweating as a way to lose weight. However, by increasing the intensity of the workout one would usually work up a sweat. A good, regular workout will lead to your higher residual metabolism. We are in agreement.
I have serious doubts that anyone could do anything one might describe as a run 4 mph. 
To the OP, I intensely sympathize with you. I’ve been working quite aggressively at my own exercise program the past 3 months and even though I’ve tightened my belt by a notch or two, my bathroom scale (on which I weigh myself nude) says I haven’t actually lost a frakking pound. :mad:
Fortunately, when I went for my checkup last week, the combination of having worn relatively heavy clothing (including jeans and sneakers) last time and having worn relatively light clothing (including khakis and sandals, which I took off before weighing) made it appear that I had lost 8 pounds on their scale. 
(Bolding mine.)
One pound of muscle weighs exactly the same as one pound of fat. 
I think what you meant to say is if you replace an equivalent weight amount of fat with muscle you will look thinner (and probably be healthier anyway) because muscle is more dense than fat and occupies less space.
Can you be a lot more specific about what you’re eating? What do you mean by a protein at breakfast? Can you list out everything you eat/drink in a typical day with (approximate) quantities? Like:
Breakfast:
10 oz Coffee w/milk sugar
2 eggs
1 medium orange
…
What do you drink? There are more calories in sugared drinks than a lot of people think. Unless you’re exclusively drinking diet soda or water, that might be a significant portion of what’s hurting you.
Thanks for all the lovely advice. Interval training I think, the trainer at the gym showed me a program she wants me to start (a freebie, since I don’t pay her) that is exactly that, more incline/speed, followed by lower speeds/less incline, more incline/less speed etc. She basically said the same some here said, that my body has gotten used to the workout I am doing. Food diary, very boring, orange, no coffee, and 1 or 2 hardboiled eggs, or 2 sausage patties when I need meat, or cereal sometimes with skim. Diet soda, unsweetened tea, and water only. And not much soda, I fear the potassium. Like I said, meal-in-a-box, mainly for portion control, and sometimes I add a prepackaged salad, which says it’s about 250 calories. All of this for convenience as well, with my work schedule. Dinner, is, well, whatever I feed the man. Tonite a pulled pork sammy on only the bottom of the bun, and about 6 oven cooked french fries. Last nite Chinese veggies (works out well, he eats the meat, and I the veggies which he doesn’t like), and tomorrow is oven baked sea bass.
I thank you all.
Add in some weight lifting to your treadmill routine, and make sure you aren’t consuming too FEW calories. Some people parrot “calories in calories out!” like a mantra, but if you eat too little your body will go into starvation mode. Also keep in mind, if you start lifting you’ll start putting on muscle, and you should eat even more calories to accommodate that.