Originally Posted by ttakata73

Search the net on setting up the deraillers.
Also make sure the cable is clamped in the derailler clamps groove.
I recently swapped my cable and there was a small bend in the cable by the rear derailler clamp that kept it from shifting through all the gears properly.
Once I put in a new cable and it was clamped perfectly my shifting was back to normal.

Originally Posted by noise boy

+1 it sounds like a cable tension adjustment needs to happen.

Originally Posted by Vicegrip

If the basic tuning items like adjusting the cable tension don't work have them check the derailleur hanger and B screw tension after they check the hanger. .