Yeah I would bring the bike right back. I know it is a hassle for you to drag it to and from, but they shouldn't have a problem taking a closer look.
Next time, try asking for the head mechanic directly. If you can be as patient as possible and just kindly ask for someone "above", it will get you a long way. Maybe if you speak directly to the guy doing the repair, things won't get "lost in translation", so to speak. Plus, he will most probably have far more experience in the matter than someone who is paid $8/hr to write tickets and answer the phones.
Anyway, from a mechanical standpoint, how many miles have you put on the chain and cassette? How old is the bike and more specifically the shifting cables? Do you have a double or triple in the front and how old is it? If any of these parts are worn beyond their standard lifetimes, then shifting could be a problem.