There are two crucial mistakes people frequently make while doing lower ab exercises:
Not keeping your back completely flat on the ground throughout the entire motion of a lower ab exercise.
Lowering your legs too far down, resulting in the lower back lifting up off of the ground.
Improper form leads to these exercises redirecting the focus away from the lower abs and towards other muscle groups. In order to correct or avoid improper form and isolate the lower abs make sure you:
Keep your back FLAT on the mat (notice in the video below my lower back never curves or lifts up off the mat).
ONLY drop your legs down as far as you can while maintaining a flat back.
Feet do not touch the floor.
Keep your core tight/stable, have control throughout the entire motion.