probably not getting your butt right in line. make sure your hips are tucked but not arched under - maybe use a mirror or a friend? alternative exercises - try hollows - you lie on your back and try to hold your feet 6 inches above the ground without letting your lower back arch up off the floor for 10 seconds, then 5 seconds rest, for 5 rounds. if you can't do 6 inches, start higher. for somehting more dynamic, try ranger crawls - get into a plank position with your hands on the floor (not your elbows), then bring your left foot up to just outside your left hand and then put it back. repeat on the right side and go for however many it takes for you to feel it in your abs and obliques.