The pulley has cylindrical weights in it that are pushed outwards by centrifugal force and squeeze the belt outwards making the motor pulley in effect bigger. The change in ratio between the motor and drum pulley gives the fast spin.
Take the belt off and put it on spin. As soon as the motor goes into spin the pulley should close up. The weights can get stiff or they can wear a ridge in the slot they slide across over the years or if the wrong belt is fitted or old one is excessively worn.
You can lubricate the pulley with WD40 and ‘collapse’ the pulley by hand to free it off. Most times this will work but if the belts OK and the pulley moves, then there is wear in the weights and their angled slots so sadly new motor.