Drop the deck onto a hard clean surface like a concrete drive.
Then get down on the left side ( not discharge side ) and get some one else to start the mower & engage the blades .
Watch where the belt jumps off.
You might need to do this a 1/2 dozen times cause it will happen quickly.
If you can not see an obvious cause get back and tell us exactly where the belt jumps.
It will also be a great help if we know which deck you have and what belt you put on.
JD occasionally change things like the size of the idler pulleys which affects the length of the belt, or the tension on the belt.