cable adj.. if it's skipping down you need to tighten.. skipping up, loosen.(that's skipping up or down the cassette, not the "gear ratio")
you are talking a multi gear? not one of the SS?
you'll almost ALWAYS have to do some minor Adj after a shop does becasue the derailleur reacts slightly to the pressures of riding and the tech can't/don't usually adj under pressure.. they almost always speed shift it up and down the rack while on the stand. It hard for them becasue most bikes don't "fit" them to ride so they can "pressure test" you bike. if you mech is my size. (5'8") he's not going to pedal around on your monster bike being that you tall ass has the seat almost up to his head....
best way to adj is to put on a stand (or flip over) put in middle rings both front and back. spin cranks at medium slow speed and turn the cable adjuster.. bring all the way up so you hear the chain clicking on the next gear up. then count as you spin down to where it almost drops onto the lower gear. bring it back half way and try riding it.. shift under pressure.. I find I usually have to give it 1/4 turn tighter with Shimano and about 1/2 with Sram systems when riding with pressure is on the system, over what it does in the stand..
if you do it this way you can get the thing to shift while climbing and you'll get much less chain "clank" that you always hear when people try to down shift when in a major climb.
if it still happens you have one of three things wrong...
bent derailleur hanger (most likely)
Bent/kinked/bad cable
bent derailleur cage
worn out chain?
worn out cassette?
One other thing that can cause this is a bent axle.. but you'll usually notice the wheel is wobbling bad or is way out from being straight in the frame. but it can cause the cassette to pitch out of angle to everything... this is very unlikely though unless you've been hitting some drops or something.
