There is so little involved that I'd be looking for a better lbs if they can't fix it after returning one more time. If I were you I would 1) buy new cables and housing, cut them and ensure there are no burrs and they are all cut nicely and replace them 2) clean and oil the derailleur and make sure it's properly aligned 3) recheck the HI/Low stops with the new cables and tension the cables. That's pretty much it. The local lbs should be able to do all this in a heartbeat. They should also have seen if the rear derailleur is not aligned but it's so sad that many lbs have some young kids doing bike work. We only have two local shops (maybe a small third I've never been to...) and both are pretty decent. You might also have bent hanger which can be replaced or re-bent on most any bike. The culprit isn't likely the cable because that would typically show when you let off tension but it's not impossible. A total rework of your cable's and housing is a 10 minute job for an experienced bike person and retensioning everything is easily done in less than 30 minutes. Bring it in clean and give them one more chance and then look for another bike shop if you can simply because it's good to have somebody competent available. Bent derailleur is indeed possible...