Did they also change the chain? If you are moving from a five cog cassette to a 6 cog cassette you may need a different chain. However, that is not likely what is causing your problem. If it is jamming between the derailleur and large chain ring it sounds like an over-shift. I'd take it back to the bike shop and ask them to make the adjustment while you watch. This way, you'll either learn how to make the adjustment yourself or learn that you need to frequent a different LBS.