I'd agree that for a new bike it's probably the cable tension, also known as the indexing. But rather than simply guessing which way to turn the adjuster, this video has great visuals on adjusting rear shifting that gives a great deeper understanding of the mechanism: https://youtu.be/UkZxPIZ1ngY 

How to fix the shifting on your bike. The majority of rear shifting problems can be solved with basic adjustments to the limit screws and index setting of th...