I can't suck my belly button in while exercising??

What sort of ab exercises are you doing? Try doing planks and push ups - the nature of the position naturally makes you hold your core tight and help strengthen your core muscles.

If youre aimng for a flat stomach sit-ups won't help you, abs come from fat loss.

Mirita, it's hard to say without knowing more about you. After I broke my back my rehab trainer kept telling me to suck in my lower abs or upper abs depending on what we were working on I found I could on my upper abs but not my lower abs.

There's this thing call muscle memory and when a muscle isn't used for an extended period of time the pathways from that muscle to the brain just don't fire right, and that muscle literally forgets how to function properly. So I had to just lay on my back with a book on my belly and focus on making the lower ab muscles work.

Why the book? Because sometimes the movement is so minuscule you don't realize your doing anything, so the book helps to make you know your doing it. After you focus solely on reconnecting the pathways from your brain to your lower abs then you'll be able to do it more and more during exercise.

Again, I don't know your particular story but this may be your problem. Oh and by the way the rehab trainer said this happened in people of all weights. Just because you may be skinny doesn't mean you use certain muscles, same as being 100+ pounds over weight there may be muscles you use that skinny people don't and vice versa. this has nothing to do with weight but simply muscle use or the lack there of.

Hope this helps.
astattonhunt