I'll have to tell you to wait for some other responses as well, but I think you may just need to increase the tension a bit. If it's shimano (possibly other brands too, but i have no personal exp with others), then there should be a barrel adjuster at the rear of the RD. Turn it a quarter turn at a time (clockwise i believe) until it begins to shift properly. Keep track of the original position, though, in case that isn't the problem and you need to go back to the original setting.

If that works, you should make sure that you haven't overtightened by checking to see if you can downshift all the way back up the cassett.


Or, you could always do yourself a favor, disregard me, and check out parktools.com