I had this happen in the same gear combination on my MTB. It truned out that my derallieur hanger was out of alignment. I ended up buying a Park hanger alignment tool and after tweaking the hanger, the bike shifted perfectly. The tool was about $40, so you might want to just bring it into a shop instead of buying one. Of course,this might not be your problem at all, but I have checked a lot of friends bikes and found the hanger to be misaglined. Like the others have suggested though, make sure the cables and housing are clean and that the shifter cable is sliding through the cables smoothly. Good luck.