Make sure the belt is following the factory specified path. If it is then either the belt is stretched / too big or the tensioner isn't working. Ensure there is tension on the belt then Watch the belt and start the car. Are any pulleys wobbling...the belt still tight. Repeat process until you see what isn't working.

The Belt Tensioner is probably crooked, it's hard to tell but if you look at it while the vehicle is running and it looks tilted even just a tiny bit, it will cause the belt to fall off. Belt Tensioner.