i would check two things. First just make sure the humidity in the paper is not to high. It can cause the paper to curl and jam when trying to duplex. Try a new ream of paper to fix this. Second thing to check is to pull the fuser assembly out of the machine, which youcan get to by lifting up the scanner assembly, and then the top cover. Check the long orange roller on the output side of the fuser unit, and make sure that the little black tab has not flipped up and causing the orange roller to sit crooked. This will cause issues as well. Hope this helps.
