Not being smart .....check the routing of the belt ie: how it goes around the pulleys. It will only work properly if belt is correctly fitted.
Also check how the belt travels around the pulleys when the deck is in its lowest position with the blades engaged (engine off), often the movable pulley is skew allowing the belt to run up the rim of the pulley.
last thing, throw the mulch plug, if fitted, in the bin.
I hate the bloody things
Good luck, let us know how you go.
Ps check the operator engages the belt engagement slowly as quick engagement can cause problems as the belt battles to get the blades up to speed quickly. This is the biggest cause of standard belts dying quickly. They are not designed to slip to allow bade engagement under load.
