Need to tweak my ab exercises, suggestions??

Hi, for a while I was doing 300 crunches a day and then kind of hit a plateau.. Since then i've been doing these other exercises:

- lay on the floor facing the ceiling, lift balance ball with feet upwards

- side crunches

- balance feet on a balance ball in a pushup position and roll the ball towards my core

- a downward crunch w/ this thing at the gym.


Lately I'm having a hard time feeling any sort of burn in my abs when i work them.. I like to atleast feel a burn & know that something is happening..
Does anyone have any suggestions for ab exercises?

Thanks!