I have seen it where pads will get electrically isolated from the fingers and give you marginal/flaky contact. you gap an clean it works for a little while then ever once in a while it doesnt... See if you can see any burning arcing around pad. The fix, the hard way, replace contacts. The easy/hack way, take a set of pliers and crush the two pads into their fingers. Ive also filed and soldered on the backs to ensure good contact.

Ken (King of the Hacks)

Om the trough switch contacts