It veries alot to do with your body ypes and how you body responds to certain exercises. Prehaps you need to begin hitting abs for 10-20 mins before every workout and start incoperating new exercises. I will begin doing this. I like to do sit up, crunches, rop pull downs, weighted crunches, leg ups and i work my obliques with a pole that i swing from side to side for 5 mins then hit them with weighted oblique pull ups (Sorry about terminology and i know the exercises but not the specfic name for some).
Remember though the more learn muscle you have the more your body burns fat. So working all parts of your body will improve to show enhance your abdominal region.