there are two things you can do here. The main reason maybe is that you are banging out loads of reps and not really feeling the muscle work. So you can really slow the movements down and get that burning sensation in the abs, try static holds in the hardest part of an ab exercise. You could also work on the underlying core muslces by doing plank, side plank, etc and by incorporating core functionality movments into your workouts. Try also adding ab movments that use weights, ie, cable crunches, weighted situps etc. Hope this helps.