most likely it is the gear linkage rod connector is out of adjustment, or in the wrong position. my mercury 115hp 4s has an adjustment screw at the top in the motor cowl.
my older merc 60hp 2s, had a cam that needed to be in a certain side of the lobe, learned that the wrong way.
My 9.9 yamaha has an adjustment with a set-screw on the shift rod.
what is the year, hp and make of the motor?