The pulley may be worn. I had a lawn mower that the drive pulley was too worn and it became very thin. Finally failed to hold the belt because it would deform so much.
Yes, as SHOZ says, the pulley (Probably the one on the engine) may be worn. V-belt pulleys need to be V shaped like the belts, but they will wear till the sides are nearly U shaped, and the belts won't grip anymore