I train abs twice a week, and same thing, I feel a burn but rarely get sore. The one exercise that I love and always gets my lower abs sore is "plyo leg raises". Same movement as straight leg raises, lie flat on the floor in supine position, grasp partners ankles, point toes, contract abs and pull legs up like you are going to kick your partner. The partner pushes your feet towards the ground, not too hard. You try to keep your feet from hitting the ground, and pull your legs back up as fast as possible. Try 3 sets of 20, alternating with your partner. If you've never done these you will be sore the next day.