I had this happen once where the balls were magnetized but that is not likely in you case.
I have also had a switch "sometimes" work in the trough area where the switch was wired incorrectly (wrong lugs of the switch) from the factory! You could also have a bad/open diode OR, MOST likely, this...
There is a type of clear plastic "sheath" that goes over the switch connections and diodes in this area of most system 11's. I would gently tug at each wire/diode/wherever there's a "sheath" to confirm that at some point, a previously disconnected wire was not just "shoved back up in" the plastic sheath to attempt a "quick fix" and not actually soldered to the switch lug or diode connection.