New Bike Frustration...

I had a similar problem years ago when I converted my double to a triple on my old road bike.  I had accidently set the derailleur too low on the tube, so it couldn't quite make the change I wanted. If you are comfortable working on the bike, just loosen up the derailleur and raise it in 1/4 inch or less increments.  If you are not, take it to the shop, and have them try that.  It could also be that the cable is not correctly adusted (or may be too loose and not be able to pull the derailleur without slipping).

In any case, it's unlikely it would get better with time all by itself.

"the guy couldn't fix it"?

Find a new LBS.  

All new bikes suffer from cable stretch after a few rides, a New Bike Tune-Up may be in order. This might even be part of your service agreement when you bought the bike-it usually is. 

You haven't supplied enough information you really guess what the problem might be. But, to answer you in general:

New bikes have a breaking in period, but typically the shifitng get rough as the cables stretch, not impossible. I am leery of a shop where the mechanic can't tell yuo what is wrong with the derailer.

x2

When you buy a new bike they will often tell you to take it back to them in a couple of weeks to sort out any minor problems, such as shifting issues.  They shouldn't charge you.