Is this a cylinder model? If so, I can almost guarantee that the roller drive gear is stripped. Remove the side casing, remove the roller drive pulley, and behind it you will see the drive spindle assy held in by (depending on production date) 3 or 7 screws, and it may or may not include the roller bearing. Remove it, and the gear (replaceble) is exposed. While it's out, examine the internal ring gear in the roller. Less common, but it does wear.
Expensive, but easy.
phil_saunders(a)bigfoot.com
Phil Saunders, September 2012
