Well it sounds like adjusting the derailleur is in order. Directions for setting the above and below limit screws - which in turn sets the chain location for each gear on indexed systems - are an easy search on the web in general and this forum in particular. And if the cassette and chain needed replacement your derailleur cable should be changed out as well before you bother adjusting the derailleur. It's not as hard as you might think and is a good thing to know. Once your new cable stretches you'll be doing it again. Good luck.

DanO