Not knowing the unit, General. check that you have not got any grease on the drive parts rollers belts etc or any surface they come into contact with, use a degreaser, IsoPropyl Alchohol (IPA) is what I use. or Meths Some times a light rub with very fine wet and dry, on the rollers, gives a better grip, and keeps the speed even.
Sheepdog, March 2011