It could be several things. You may need to burn at a slower speed. It could also be the media you are using, slowing down may help. It could also be a defective burner (or one that is failing). Or, it could simply be a burner that needs cleaned.
Sounds like a lot, but really isn't. Just try to slow things down first to see what happens. If that fails, you may need to change media brands for the blanks themselves. You could also get a CD disk cleaner and run through your burner. If none of these work, you likely have a burner that is fading. BTW, are the disks usable after the burn fails?
Doesn't sound like a Nero 9 problem. Last, what OS are you using?