Your problem could be one or more of many things.

If you have the means i would do the following:
-make certain it is not a cable tension adjustment that can be fixed by the twist of a barrel adjuster
- if the derailleur is not returning (up-shifts) properly it could be your cables have a bit of gunk in them or the pivots of your derailluer. in that case recable/housing your bike is always a good idea also clean/lube those pivots
-if thats no good make sure that your hanger is strait
- also make sure your chain is not too stretched
-if it is take a look at your cassette because that could have gone bad depending on how long you have been running a stretched chain.]