Welcome to OCC
Pictures would help a lot in fixing your problem. A picture from the side of the mower deck showing both the deck and the belt angle in relation to the deck pulley and one from the front showing the mule drive ( that's the part that turns the belt 90 degrees to run to the mower deck from the engine)
It may be as simple as the front of the mower deck being adjusted too low.
Are you using a Cub Cadet belt? ( any number visible on it? )
Jeff