Just sounds like your ground wire is broken or disconnected somewhere. Use a meter to trace it down.
It's between the button and the relay on the CPU board. Probably not he relay itself since the other flipper works fine.
Look at page 17 of the schematics here: https://www.ipdb.org/files/2242/Williams_1985_Sorcerer_Instruction_Manual_with_schematics.pdf