Things to check on these if it won't self propel....when you pull the drive lever, this operates a lever on the gearbox (on the rear axle) to pull it into gear. Check that it does this. Also, if the axles turn but the mower still doesn't drive, the plastic gear teeth within the rear wheels fail too. The tension on the belt is kept there by a spring which tugs the complete rear axle, and its worth looking to see if this is keeping the belt reasonably tight.
John, May 2009