Originally Posted by
FBinNY
We'd all also be guessing because your description is so imprecise. I have no idea which is the first pulley, or whether you're referring to either pulley or maybe the 1st sprocket of your cassette. Please reference this
photo glossary with the standard names of the various parts, then repost clarifying exactly what the problem is.
The chains slips off the upper pulley on the Rear derailleur (Just after the chain leaves the cassette the chain falls of the upper pulley on the RD). The lower Pulley has no issues. Also it is not bent.