There could be several reasons for the lack of activity in your feeder
(1) The presence of predators nearby. Do you have cats hanging around your yard? They will chase off and/or kill birds if they can.
(2) Plenty of other food available in the area. If the birds can find their favorite food easily, they may not be attracted to your food.
(3) Wrong kind of food for the birds in your area. I know PapaBear endorses the generic seed mix, but in my opinion, it’s mostly filler. Very few birds eat the millet (the BB-like seeds). Try black-oil sunflower seeds, or thistles. They’re both expensive, but birds love them.
(4) The seeds have rotted. If bird seeds get wet, they will go moldy and rot. Dump it out right away. Moldy seeds can actually kill birds (another reason why you don’t see any–they’re all dead!).
(5) Bad location. If the feeder is still 3/4 full, that means even squirrels haven’t tried to go near it. Those tree rats will eat anything. Since they haven’t, I’d have to assume there’s something bad about the location (too noisy/busy/close to people/etc). Try another location.
What kind of feeder are you using? Each one is best for a certain kind of bird. Hanging feeders are good for sparrows, wrens, doves, and perching feeders. Suet feeders are good for woodpeckers and sapsuckers. Tray feeders are good for ground feeding birds.
It takes time to attract birds to a new feeder. Remember, it’s not like birds are flocking together to tell each where the good feeders are. Try putting out a water source too. This time of year birds need water more than they need seed.