Aside from the obvious like miss adjustment, has your derailleur hit something and bent the hanger? To check this, shift to the middle cog then look at it from behind. The chain should follow a straight line from the cog through the jockey pulleys of the derailleur. If not you might have to tweak the hanger. Also check that the hanger is not loose. I had a loose derailleur hanger that drove me nuts trying to adjust the shifting until I found the real issue. Tightened the 2 little hex head bolts holding it and it worked like a charm.