Ab definition is 90% bodyfat % and 10% ab work. You can do 5000 crunches a day and you won't see your abs if you've got a layer of fat on top.
If you are already lean and want to hypertrophy your abs, try weighted cable crunches. I do mine standing like this:
The trick with all ab exercises is to consciously squeeze your abs as hard as possible on every rep. You can't just go through the motions, otherwise your hip flexors and other muscles are going to take over.