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...
".. 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)
1984 Fast/Nicholson 345 "FastForward"