from your comments it sounds like it's a front derailleur issue you are talking about, or at least a front shift issue. If your old chain was badly worn and it was replaced along with the rear cogs, it's not impossible that your front chain rings are worn and need replacement. your lbs should have seen that but maybe they didn't. a badly worn chainring is fairly obvious. The teeth would no longer be nicely formed and the "U" shape would be elongated. You can find pics of that kind of thing with some web searching. If the issue is in the rear it's a matter of adjusting the cable tension.