The grip band comes off the ball when I am spinning really fast...

Fault: The grip band has become stretched or oiled and slides around.
Solution: Wash the grip band in warm detergent based water.
 
Detailed Explanation: Using a Powerball over an extended period of time can result in the [otherwise tight] rubber grip band which runs around the spheres' equator becoming loose and sloppy.
 
Whether this actually happens (or not!) is based on;
 
1. How sweaty your hands are in general
2. How powerful your actual grip is
 
If your hands are naturally clammy/sweaty then, over time, these body oils will penetrate the band and cause it to loosen its otherwise sticky grip on the surface of the sphere.

If this does occur on your Powerball then the simple solution is to remove the band and put it into the clothes washer with the next warm or hot load, or into a basin of hot water with some grease eating washing up liquid - leave it there for at least 15 minutes before washing and then rinsing thoroughly with fresh cold water.

After dabbing dry with a towel the band should have the nice, ‘sticky’ characteristic it had when it was new!

NOTE: While the band is 'soaking' you should clean the outside of Powerball with a damp cloth and a small amount detergent, this will give the band a fresh surface to grip onto when it's ready to go back on.

Likewise, if your general grip is poor (this will improve by using the Powerball by the way!) then you will find that the ball squirms and wriggles in your hand as the inertia builds, causing the grip band to slip upward or downward on the sphere. Over time, this will serve to loosen the tightness of the band and it will become sloppy in its ability to fasten itself to the sphere’s surface.
 
Once again, a quick boil in the washing machine will return the band to its original tight state!