1998 Dodge Caravan Serpenteen belt keeps falling off

I keep losing my serpenteen belt and cannot firgure out why. I have replaced it at least 4 times in a 2 yr period. Recently I replaced the pulley. The belt will stay on for a few days now and then falls off again. I am wondering if something like the bolt on the tentioner pulley is stripped and loosening making the belt keep falling off. Any suggestions would be great!

thanks