My Stomach Won't Get HARD!!!!!!!?

I'm 22 years old and 187lbs.....I pretty much am a fit dude, i go to the gym 4 days a week, with 30 minutes cardio before and after a weighted workout. my arms got very big muscles, and my chest is big in the muscular way........but my stomach will not go hard, i don't know what i'm doing wrong!!! I try to do a good amount of sit ups and a machine called the torso twist. My diet is strict, i eat a alot of chicken, vegetables, meat, eggs NO SWEETS WHATSOEVER.......although i will confess by saying i'll occasionally have a bowl of cereal at night (round 10ish)....but its completley healthy (e.g kashi). I did lose 60 pounds 3 years ago, but my stomach seems to be the last one to go down fighting. Now even though I have tried Atkins, and hated it (due to the fact that i was constipated) should i give up a month and just do it, but... If I DID do it..........will i lose muscle tissue in my arms and chest?....or do i need somethin wacky like a colonic? all help is appreceiated!!!!!