I've had similar situation with a Hurth gear.. it seemed the cam on the shift lever didn't fully engage the slider that moved the plates internally. I removed the lever/cam assembly and had a welder build up the cam a bit and it worked for years again after that....
Different gear, though...
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)