Mercury 120HP outboard loud clicking noise when shifting into reverse

[Waternut567]

Hi all, I am getting a very loud clicking noise (sounds like gears not quite engaged) when I shift into reverse. I disconnected the cables and tried shifting it by hand. The shifter moves about half way into gear and then won't go any further. It moves in and out of forward with no problems.

Let me know if anyone has any ideas of what the problem might be. Thanks.