I found some images of the internals online. From the look of things,the design actually uses a piece of sheet metal with a perforation cut out as the 'switch'. I need to verify though. I'm going to do some tinkering and see what I come up with.
Check it for oxidation. If there's rust or black oxide on the steel (for copper this would be green or brown), you may need to lap the surface with 600 grit sandpaper. Then apply dielectric contact grease to the surface to prevent further oxidation. You can get a small tube of the grease at any auto part store. People use it on battery terminals and light bulb fixtures. Only use a very thin coat to cover the contact area. Avoid contact with plastic parts if possible. If you get it on the plastic, wipe it off and clean up with alcohol. Otherwise it may degrade the plastic with time.
Grease lasts a long while but may harden over time and increase resistance, so you may have to clean and retreat the contacts every 5-10 years or so as a routine maintenance.