I cant burn past 8x

I have a 15 inch powerbook with a superdrive thats supposed te be able to burn at 24x, at first i could but now it only gives me the option of 8x and every application makes my highest speed 8x when it should be 24x, is there any way to fix this problem!

Are you burning CD's or DVD's?
In iTunes for example, under preferences, I have a drop down box that is normally set to 'Maximum speed'. If I select that box, I have the option of manually choosing 24x.
If I've got it set to data dvd however, the max. speed option drops accordingly, due to the lower burn speed of my external DVD burner.

Burners also auto-detect the speed of the media you're trying to burn on. If the media is 8x max then that's the max you'll get.