Why can't I suck in my lower abdomen, no matter how hard I try I just can't suck it in, I'm not fat, but I use to be able to?

Core Strengthening. The most likely reason you can not "suck in your lower abdomen" is due to laxity of your muscles. You can firm up these muscles by doing core strengthening exercises. Lay on your back with your hands behind your head, put your legs straight up in the air so you are at a 90 degree angle, tighten your buttocks and pulse you legs upward, hold for 5 seconds and repeat 15 times.