drive belt keeps slipping off my washing machine?

why does the drive belt keep slipping off my Hoover washer dryer,wd120?
i have changed the belt but its still the same.when i put it on and turn the big wheel on the drum i can feel it starting to creep off,then pop,it slipps off!
the motor still seems to be in the same place,nothing seems to have moved.no strange noises etc.