Everyone so quick to judge and blame the bike shop.
How many miles did you ride the bike before this happened? After the first ~10 miles, the cables will stretch and cause a bit of play in derailuer adjustment. Even pre-stretching the cable, some play develops.
It could simply be how you are shifting. Rider style and preference plays a BIG role in component adjustment. The way I shift and ride will be different than how one of my customers shifts and rides. They complain about the chain not shifting up, but the way I ride (and the correct way to shift), the bike shifts fine.
Other problems could be mis-cut rings (happens occasionally). Chainline (crossing the chain line). Shifting under heavy load (whilst bashing uphill). Incorrect BB size. Worn rings. Old chain on new rings. There are hundreds of possible problems, the shop is VERY low on the list.
To measure the correct chain length, take your new chain. Wrap it around the largest cog in the back, and the largest ring up front. Bypass the derailuer. Take that length, add one full link and put the chain together. Perfect chain length
. It's how PARK and Shimano both recomend checking chain length.