I’ve made my own yogurt several times. BTW, if you’ve never done this, it’s fun and easy and makes a luscious creamy product ready for making into raita or pita sandwich sauce or whatever.
Here’s the deal: Last week, I tried some commercial Greek strained yogurt. This is yogurt which is drained until it’s the consistency of mascarpone, that is, nicely stiff. It’s heavenly with honey drizzled on top. I used some of this commercial yogurt to make my own yogurt, and it turned out great. I drained my homemade stuff, and had an exact duplicate of that yummy stiff Greek stuff.
Yesterday, I took half a cup of my last homemade batch and added it to a gallon of warm milk, and put it into an incubation chamber, same as always. Results: barely thickened milk tasting vaguely yogurt-like.
What happened? Do yogurt bacilli degenerate or lose their mojo after a generation or two?
I’m not sure whether it’s also a generational thing, but you may be dealing with a dead culture here.
(Wow, that sounds really deep if you apply the same sentence to concepts other than yogurt.)
Did you boil the milk or just warm it? I tried to skip the boiling step once, and got something like what you describe. I looked around online and found that boiling the milk does something important, protein-wise . . .
Quite the yogurt guru, ain’t I? 
Also, did you use your drained yogurt as the culture? Maybe there wasn’t enough moisture for the bacteria to thrive?
I just brought the milk up to a little more than blood heat, but this is the way I’ve always done it. However, if you say that bringing the milk to a boil first can help, I’ll try that next time.
I didn’t use the strained yogurt as the culture; I saved some of the thinner, regular yogurt for the purpose of using as starter. I figured it was jam-packed with bacilli, but maybe there weren’t enough to get a party going. BTW, I use a picnic cooler as an incubation chamber, and just put a big container of hot water inside as a heating source. I replace the hot water once or twice during a 24-hour period, and it has always worked before.
Oh, well, there went a gallon of full-fat Clover Stornetta. I’ll try again with the boiling technique. Thanks for the suggestion, Podkayne. It sounds like you make your own yogurt, too - do you have any special equipment?
Well, hrm… it’s been a -really long- time since I made any, but I used to make homemade yogurt all the time, from raw milk yet (unpasturized). I don’t know why it didn’t work for you this last time when that technique has worked in the past, but here is a link to several techniques for homemade yogurt without a specific appliance.
Those food dehydrators (the ones that they used to hawk on infomercials) will make yogurt; you could probably get one at any kitchen appliance store or large department store. And there are also machines specifically for making yogurt, I think–they probably run under $40.
The Greek yogurt most certainly had a culture to it; I think maybe the milk wasn’t warm enough. When I made it, I used to bring the milk -almost- to boiling–if it’s too hot, that will kill the culture, I think. (LOL I did have many batches that didn’t quite take, and that was my usual reasoning as to why they didn’t, at least!).
Good luck! Home made does taste so much better. Especially at room temperature instead of refrigerated. YUM!