Mower deck RER mower belt won't stay on

I have a series 3 33" RER. It's old and beat but always starts and cuts pretty well. Last season the mowing deck belt would occasionally jump off. Not a big deal putting it back on. It got worse and the last mowing day of the season it jumped off several times and I parked it. Pulled out the 21" walk behind for the first mowing this year and was quickly reminded how much longer it takes to cut with that. Looking for advice on what might cause the deck belt to come off. It's a snapper belt and I think it's tentioned okay. The deck has been bent over the years but I have beat it back into to shape and the belt has generally stayed on in the past. I appreciate your help in advance.

Are you missing the keeper that mounts below the tension pulley? If it's missing or bent significantly, it can allow the belt to flop off, especially if you have the blade lever to the rear.