Define "can't do it". What types are you trying to do? Is your neck hurting?
Try these kinds:
1) Lie on your back with your arms at your sides and raise your legs in the air. Bend your knees and while keeping them in the air, straighten them out. Do NOT put your hips or legs into this; work strictly with your gut muscles.
2) Lie on your back, one hand behind your head, legs in the air and crossed at the ankle, and reach for the opposite foot with your hand. Again, focus on the gut and keep your gaze on the ceiling--that'll help your neck. Then switch hands/ankles crossed.
3) Boat pose in yoga. Balance yourself on your sitting bones, and raise your legs in the air, shins parallel to the floor. If this is good, then hold your arms out in line with your legs. You should definitely feel this in your abs, and if you can take it further, raise both legs and arms.