Before you worry about voltage, I would start with the simple solution which is rebuilding the flippers, auto plunger and VUK (vertical up kicker). All 4 of these are basically the same thing. You won't find a handy VUK or Auto-plunger rebuild kit like you will for the flippers, but it's mostly the same parts. Coil stop, sleeve and plunger. My guess is that they have never been changed and are badly mushroomed causing them to be underpowered.