Whenever possible, add resistance to your ab exercises with weight. Most people increase the load on exercises for major muscle groups but they don't on ab exercises. That's a mistake. Think ankle weights for leg raises or a 45-pound plate for crunches as shown here.
It's very apparent to me that there are many coaches and fitness writers who don't understand the nervous system. I'm not the least bit surprised because your nervous system is arguably the most complex and ambiguous system in your body.