Well, given your belt is tensioned proper and you have the correct belt; I would first check that pesky belt guard that tends to get bent. It may be levering the belt off. If that is OK, check the sheeves for string or vines wrapped aroung their grooves. I don't think there is much chance of the idler pully being bent out of alignment. When ya figure it out, let us know. WELCOME to the fire!
I had a person that worked for the dealer load my tractor and never said a word but he rammed it into the front of the trailer and bent the drive pulley mount and I got it home and tore up a drive belt had to fix ir myself but they replaced the drive belt. When it's hooked up see if it looks lined up if not it's your problem.... Jim