The ammeter is just another link in the system. If it got corroded and there were poor connections, the resistance converted some of the electricity flowing through into heat. That could hae been sufficient to loosen the metal to metal press fit between the plate and the studs.
The needle doesn't touch the metal plate. It reacts to the magnetic field created by the current flowing through the plate. So the needle can still work, even though there is a poor connection.
Get another meter, or if you want, some people have fixed these. First thing is to make the stud connections to the metal plate solid. Then make sure the insulation and studs are solid.
There's a post showing how someone did this. I'll try to find it.
