Mike,
Sounds like it's a mechanical issue with the pantogram mechanism behind the nameplate. Decent DP-1's aren't too expensive, if the OP doesn't feel comfortable tearing into the mechanism to see why the pin won't engage. My FTn finder doesn't like the triangular coupling shoes used on some of the older non-AI Nikkors. Could be an alignment problem, since the only lens I've tried that has one, is my 35f2 Nikkor-O, which, at one time, in my infinite wisdom, I converted to said coupling shoe. Probably didn't get it aligned properly when I did the conversion.
-J