I have the same problem with my powerball 350.
The grip ring starts to slide away at approx 5000 rpm. Varies a little
with different ways to hold it.

I tried the washing routine. No joy. Then tried wiping the band and shell
off with rubbing alcohol. No good either. No band but rubber gloves. The gloves start to roll at about 6000 rpm.

May be start looking at some of these 'lotak' materials. That is the stuff
they use on flat sceens protection as on cell phones.

At the moment I hold the ball rotor inward to palm.I have fairly hard hands
from canoeing all summer. Which is why I have the power ball, to keep the muscles in shape over the winter.