IMHO the chest is one of the most stubborn muscles on our body.
You have to literarly beat them into submission, and use every muscle fiber. I would suggest incline if you want to bring them out try 4 sets, heavy and HIGH reps, pyramid that bad boy. then Decline the fill em out 4 sets again HEAVY and HIGH reps. Then follow it up with some flats, 5 sets Heavy and HIGH if you still can.
if you just do flat bench you wont see all that much development that fast, well at least i didnt. You have to hit chest from every angle possible, when your cooling down, or just about to finish your excersise, do some pushups just to make sure you hit em up right.
This is what i have been doing and i have been seeing substantial gains in strength endurace and size.