If your problem is that your lower abs aren't as visible as the upper ones, then that is simply a matter of body fat percentage. Most men tend to retain more fat in the lower stomach than the upper, so to have the lower abs fully visible, you need to get your body fat percentage quite low. If your problem is that your lower abs don't get as sore as your upper abs, then that is misleading. Soreness is not a good indicator of the effectiveness of your workout and does not mean your lower abs aren't getting worked.
Finally, if your problem is that your lower abs are weaker and it is affecting your performance in some activity, then here are some exercises that will help:
* L sits
* L pull-ups
* Hollow rock
* Knee raises or knees to elbows
* Leg raises
* Leg raise with l sit pauses during each rep
* GHD or roman chair sit-ups (start very light with these as they can produce debilitating soreness)