Belt failure 29 hours on machine

My belt was broke in correctly. Did about 10-15 hours of very light riding on it. Started getting some weird problems out of it this last weekend. Felt to me like the secondary wasn’t shifting the sheaves correctly. Brought it to Dothan power sports and they blew the belt driving the machine. Told me they couldn’t find anything wrong with the machine and are now charging me labor and money for a new belt. What gives? I just inspected the belt prior to bringing it in. There were no problems with it.