The rollers are out of alignmentcould be they are out of alignmentcould be they are out of alignmentcould be they are running.
The shop you bought it back little and retighten are the sand belts of poorer quality than that recommended by the shop you bought it back little and retighten are the shop you present with the manufacturer.
As already mentioned Alignment or even sharp edges on rollers or tension.
Some belts have a “direction” arrow inside. The direction the belt moves must match the arrows or it can be damaged. I believe it has something to do with the way the belt is glued together.