I have an E-mu electronic music keyboard that was damaged when someone
spilled a drink on it. Four of the keys stopped working after the mishap.

The keyboard works, I believe, by the same principle as a capacitative
computer keyboard. When you press on a key, a black circular part
plunges against a circuit board which has at the point of contact a
corresponding pair of interlocking metal plates (embedded in the board).
I believe the interlocking metal plates are acting as a capacitor, and
the black circle is some kind of insulator that gets between the
plates and changes the capacitance. There is a strip of 61 of these
capacitors on the circuit board, one for each key on the keyboard. They
are all connected to diodes to form some sort of matrix. When a key is
pressed, the keyboard senses the change in capacitance. In any case,
that's the best of my understanding.

So after the spill, I opened the keyboard up and cleaned off the residue
of the drink from the board and the black circles with isopropyl alcohol.
After that, 3 of the keys started functioning again. However, the
fourth is still malfunctioning. It works occasionally, but most of the
time, it doesn't work. I've tried cleaning it more, but to no avail. No
amount of cleaning seems to be able to improve its function.

I've found by fiddling around that if I change the angle at which the
black circle plunges onto the capacitor, the key works. Unfortunately,
it's not possible to change that angle when the keyboard is assembled.

My question is, what could be going wrong? What happened to that
capacitor when the drink spilled and what could I do to fix it?

Any ideas greatly appreciated.