How about the model number of the mower so we can see what belt you are having problems with. I have a customer with an older craftsman mower that we had been fighting the deck drive belt on for years. turned out that the part diagram had the wrong part number on it. It was calling for a 1/2 inch wide belt which worked, but actually needed the 5/8 width belt to be correct.
the 1/2 inch width belt would operate about a season and then bottom out in the pulleys and start slipping.