I would typically check: 1) Derailleur hanger alignment. Also, is the derailleur sitting flat on the hanger? 2) New cables? Check burrs as usual, but also check if the housing is sitting all the way in the ferrules. 3) Assuming you have the correct chain width and length, check for a bent or tight link. 4) Is the cassette seated properly? A friend of mine had the same problem and it turned out to be a spacer issue. Luck.