I had a similar issue as yours and it turned out that the current owner did not tighten the nut which holds the wire to the ammeter. It was just laying there conducting some electricity most of the time. I guess over time, the terminal began to accumulate some tarnish and maybe some soot from arcing to the point that it stopped conducting electricity consistently.
BTW, that "current owner" is still in place. His wife considers him quite the catch despite his occasional mechanical ineptitude.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission