Dirty chain,
Stretched chain,
Shifter cables need cleaning and lube ? Or do you need new ones ?

New bike, cables are stretching In,
If all is clean and bike is just at four or five rides the barrel adjusters on the shifter ends can take up slack giving you back precise shifts.
Counter clockwise a few clicks at a time while you shift and ride should make a change.
Remember, only one or two clicks one way then ride a while..