Odds are your belt and/or pulleys are glazed. Scuff the glaze off all the pulleys with something like a coarse scotchbrite pad, always worked for me.
I wont use WD40 on belts, it has petrolium distilates and that isnt good for them. Besides you want to clean the surface to stop slipping, not lube it. There is a "belt dressing" made but IMO its a short term solution not a fix.