This sounds like a common case of cable tension adjustment. Try adding 1/2 counter-clockwise turn at a time to the shift cable barrel adjuster found where the cable enters the rear derailleur. You may need more than 1/2 turn so try again after riding. It's very common for shift cables to stretch or actually "settle in" on new bikes. The original LBS should adjust this at no cost but since you have moved the bike across the country I guess you won't be taking it back to the dealer.
Detailed instructions for derailleur adjustments can be found at http://www.parktool.com. It is important to do all of the adjustments in sequence. But if you have not messed with the limit screws you can probably fix your problem with a simple cable adjustment. Your bike is way too new to have a worn out cassette.
Checking for a stiff link is a good idea but I don't think it's your problem. The bike is too new to have a weak derailleur spring.

Al