If the bike is new, then the odds of the chain being stretched are small ...
The possibility that the (rear) derailleur's cable has stretched is more likely ...
You can EITHER bring the bike back to the shop for maintenance (which they should do for "FREE" for the first six months to a year ... some shops will make MINOR adjustments for free for more than a year) ... OR, do it yourself.
Presuming the 'shop' set up the indexing properly, you simply need to tweak the cable's tension to
restore the indexing ...
You can check the PARK TOOL website (
http://www.parktool.com/repair/readhowto.asp?id=64) for some maintenance tips ...