The boards themselves. I'm not sure what you mean by sheets that go between them. I'm guessing your board assembly is in an EM cage like many games of the era.
Your coins not working can be because of several reasons - bad switch, bad wiring, bad inputs on the mainboard - Check the switch first by taking a piece of wire and jumper the leads on the switch itself. If it suddenly credits you have your answer. If not, carefully examine the wire leading back to the PCB.