If the other bells are working but not sounding well or reliably, check the plunger - my Gottlieb duotron, the plungers were all gummed up. Alcohol wipe of both the plungers and the sleeves brought them back to life.

And not sure of the conditions of the rings / rubbers - on another game, the slingshots felt all loosey-goosey / sloppy but a new set of rubbers set improved things immensely. About 1/2 the time I'm looking for bad contacts / wiring issues and it's usually just cleaning, maintenance sort of stuff.