OK C, ?I guess I am beating a dead horse, but:

I am one of those guys that just gotta know exactly why something fails, along with the failure mechanism[s].

In some of my working days you couldn’t just throw out a 2000Amp bus-bars because current didn’t go from one end to the other; you had to find the problem [the real problem]!

You triggered my 1950’s curiosity. I thought I knew what a trimmer is/was, but I never built one, so I guess I have been missing something.

I have not been able to find details of trimmer construction; only simplified sketches.

So we can only presume that ?if one plate of the capacitance part of the unit is one ?metal piece and it has fractured [maybe due to adjustment]? or

[ more likely] if one plate of the capacitance part of the unit ?is SOMEHOW connected to the terminal ??metal ?using a simple compression means and that compression has weakened [maybe due to normal thermal cycling] or, as likely you are saying, corrosion wiggled it way right between the pressure/ contact points, which is all aided by the local climate conditions, and that corrosion is non-conductive [consistent with aluminum oxides]

Re: “I opened it up and it appears to be corroded.”? ???Is that a one way operation, or can it be reversed? visible corrosion should not hurt, but if there is a joint …..? ?

Maxwell did not allow for bad joints in his formulas.