Since aluminum is a poor conductor obviously his could be an issue, as could corrosion. We doubt that as usually it is the bolt mounting of the ground wire through the hinge mounting bolts. The ground on the mounting bolt stem offers some play in time with that bolt mount. That play tends to become worse in time and the ground deteriorates from wobbling machine screws, loosening of hinges, wear of carpeting, rubber neoprene capped seal washers wearing and many more areas. Tighten the machine screws up and revisit the mounting of the ground wires on those lids which seem to have developed a problem. You could even extend the wire to a direct mount inside the lid, as long as the mercury switch is functioning. A very few of the mercury switches did get overheated when the pockets were welded to the lids which caused solder joints on the single one wire to fail.