How long have you had the mower ?
there was an upgrade to some CC decks a long while ago
They fitted bigger idlers with bigger flanges and changed the height of the fixed idler from memory ( which is never totally reliable ) .
You really need to get down on your knees and watch the belt to see where it is jumping off.
It does not take much, a slight warp, a little twist is a lip or a pivot that is not true to the deck will cause a belt to jump off as well as the other things menioned,
At times I have resorted to putting a Go-Pro under the deck while mowing to find the problem .