Sometimes the serrated lock washer, if used... can cut through the foil, or paint, reducing the effectiveness of the contact... so a link of stretched TCW is a good call, or run all grounds to the Jack... easier with a simple single pup less wires... dont forget the bridge earth.
EDIT: Sometimes the serrated lock washer, if used... can cut through the foil... Meant the foil on the back of scratchplate, not copper foil lining of a cavity...