I have the same problem with my belly. I found this weekend that the problem may be the Transversal Abdominicus (TVA).

This muscle takes care of the flat belly we all would like. You can overtrain your other abs while this one stays weak, so your belly will stick out like a beer belly.

I start with special exercises for the TVA.

See bodybuilding.com for these exercises, search for ´vacuum´.

Stop your crunches!