i saw my leg development really get a lot better after i stopped trying to lift heavy with them. used to do squats at 6-8 reps, was strong but the size and shape never really went anywhere. now i concentrate on high reps.. 15-20 usually, super setting each exercise with another. also when it comes time for extensions and curls.. try doing a drop set and not stopping until your in tears.
also.. the problem could be with your diet. if you want bigger legs, or need your legs to catch up with the rest of your body then eat bigger on leg days.