Do you do Lower Ab Workouts in your six pack abs program? No? Hmm. I think there are three kind of people trying to build six packs abs out there:

Those - and this is the vast majority - that will never get what they want. They are simply to weak to stick with the hard routines including workouts and diets you have to do over a decent period of time.
The second kind works out hard, loses body weight following a diet and finally gets some ab muscles showing up. They probably needed weeks to reduce their belly fat and now discover their abs being kinda imbalanced. Spending a lot of time doing crunches and sit ups made them pretty nice upper abs, but they totally ignored doing exercises for the lower ab region, so they developed a less pretty four pack.

The third kind - and you hopefully belong to this group - is doing some lower ab workouts at least 30% of their whole abs program. Once your belly fat is gone, your real six pack will show up.

So I highly recommend using lower ab exercises like leg raises from day one of your washboard abs project. It will pay off with a real six pack (or even eight pack) in the end!