Some people say that ceramic balls cause problems as due to being so hard they don't grip the plates as well as carbide. This is a very good thread:
http://www.rctech.net/forum/electric...er-clutch.html
Try using carbide diff balls and ceramic thrust balls. I changed mine from the stock carbon chrome
to tungsten carbide the other day, i've bedded it in and run about 3 packs through it and it seems OK.