Those things are just no fun. The game I just restored has four of them. The ball count lights, the lights under the upper rollover guides, and one above and below the score reels. You can just simply count on them being flaky.
You can clean them but that will only take you so far. They have gotten loose in the frame and they lose ground. Rotate both the socket body and the solder tab around and eventually you'll find a spot where they will light.
I would submit that those things were not one of Gottlieb's better ideas. If you really want to tear your hair out, take a look at a game like Sure Shot that has the pool ball rack on the playfield and all 15 of the lights are in one of those plate assemblies. Of course, again, they didn't expect us to be trying to restore and play these games 50 years after the fact, so they weren't engineered for that.