Do you have the idler pulley engaged?
Is the tension spring on the idler pulley stretched or broken?
Did you buy the correct size belt as called out in your owners manual or did you take the old, worn out, stretched belt to the store and have them match the size? If they matched the length using your old belt, you probably bought a belt that is too long.
My 2 Cents,
Good Luck!
Dodgeman