Have Boat Questions? Ask a Boat Repair Expert.
If your control cables are adjusted properly then there is no adjustment to the shift system.
Put the control in neutral, have a helper spin the propeller, it should be free with no clicking. If there is clicking the control cable needs adjustment.
For your problem, to shift the engine must be at the slowest idle speed. If it is too high it will make shifting difficult.
More likely the shifter in the lower unit may be excessively worn. If you hear a clicking as you put the engine in gear what is happening is the ears on the shifter are "ratcheting" in and out of gear. Once they are excessively worn the lower unit doesn't want to stay in gear. A little more throttle once in gear will usually keep it in gear until too worn. If this continues then the only fix would be to have the lower unit taken apart and the shifter and possibly the gears replaced.
I would also recommend checking the quality of the gear oil. Check for any metal on the drain plug. If slivers of metal or more than just a tiny bit, then that is telling us it is excessively worn.
Post back to what you find and we can go from there.