Take a look from the rear and see if the wheels in the dérailleur cage look like they are in a line with the gear that they chain is in. If the line is skewed but parallel, look for a barrel adjuster on the cable as it goes into the dérailleur and bring it in line using the adjuster. For most setups, you will want to put in a counter clockwise turn to put a little more tension on it. Cables do stretch a little. If no barrel adjuster at the dérailleur, check for one on the back of the shifter where the cable exits.

If the line is angled, something is bent.

Any shop should be able to help you out. I've never charged anyone for a single dérailleur adjustment.