Is this a new belt? Sometimes the belts will look fine, but they are very stretched out and loose and need to be replaced. I also keep spare belts on hand so that I can replace them every year on my own vacuum cleaner.
If you can't find a belt locally, then check with the vacuum shop in my town, they ship out repair parts all the time. They have a web-site at www.APlusVacuum.com or call them at 704-821-3863. Ask to speak to Rocco, he's a great guy.
Roger
December 2006