just like all the other muscles in your body, you can become very adapted and used to a certain set of moves if you do the same thing every time. with abs, you need to work on doing a wide range of different moves and variations, which you can easily find out about in books, on the internet or by asking a trainer to show you a few. this makes sure that all the different areas of muscle in your core get a workout, not just the same parts every time.
you should also vary the amount of reps you do, increase the number by 10 or 20 (or whatever you can manage) at every work out. theres no wonder you don't 'feel the burn' any more, because your muscles have adapted to the workout and aren't being made to work any harder.
also, make sure that you contract the abs and pull them in tightly (as if pulling the belly button into the spine) when performing the moves, and really focus on keeping that held, because that ensures you are training the abs to flatten and become tighter which is what you want. 'squeeze' the ab muscle tight when you reach the crunch part of the moves, and you should soon start to feel that burn again. with abs its far more important to focus on form rather than quantity of the moves. focus on trying to do your sets with perfect form, before you start increasing the reps, as form is often where people are going wrong.
try doing a few of the moves in each set really slowly. this is a good technique as it really makes the muscles in your abs work hard and longer, and also forces you to think about your form. many people make the mistake of just rushing through their ab workouts, and end up using the momentum of the movement rather than the actual muscle to get the moves done.
so in conclusion (sorry for the essay!)
- use correct form
- do a wide variety of moves
- increase the number of reps progressively
- slow down the speed of some of the moves
hope that helps you, and keep us posted on how you get on. good luck