Foreign objects (small stones)cutting the belt.
The pulley not parallel,belt running out of the flange.
Pulley ribs' height incorrect.
Solution
Remove the foreign objects and replace the pulley and belt.
Calibrate the parallelism of pulley.
Replace belt and select qualified pulley.Industrial V Belt factory
website:http://www.pioneerdrivebelt.com/v-belts/industrial-v-belt/