Put new belt on it, ran fine for about 45 minuetes, and then it eats the belt, pulley running slow

Depends on a few things.whats the tractor?how many pulleys?is it the correct belt,and on right?mine had a barring out in one of the mandrels.my father in laws was a idler pulley.keep in mind if it's the mandrel,you should get the pre-built one's.it's almost a 100% chance the mounting bolt's will snap.they are self tapping screws.it should be a quick,easy,and rather cheap fix.well at least cheaper than a ton of belt's.best of luck to you,and I hope this was food for thought.