Double-check that the ribbed side is riding on pulleys with ribs and the smooth side is riding only on smooth pulleys. Suspect the spring-loaded tensioner is stuck. Usually you can pull them back with the ratchet or breaker bar. Once you do, there's a 50/50 chance it will move freely in the area it needs to keep the belt tight. If it doesn't maintain proper tension on the belt, replace the tensioner.