two common causes on Yanmars. The first is the use or automotive spare belts of the wrong size - they look the same cross section but they arent. Cant now remember the precise difference but its something like the auto belt is half inch and the Yanmar is 13mm - either way using auto spares allows the belt to sit at the bottom of the pulley groove rather than grip from the sides and it slips.

The second is the pressed steel Yanmar pulley coming apart / becoming loose.