Also make sure you have the correct tension on it. The spring should be stretched a bit. The manual has the measurement from mule drive to center of pulley 3 or 3.5 inch I think. And the new belt will stretch after a few mowings.
One other thing to try is reverse the belt so it is pulling the opposite direction. I had trouble when i switched from snow duty to mowing and it would throw the belt. Some belts like one direction better.