1. Make sure the slider (if so equipped)
on top of the switch is set to
regular operation instead of 2-train operation.
2. Turn the switch over. On the bottom you will see a metal plate with 3
small screws. Remove the screws and the plate. You will see two metal
contact plates with a wiper that slides back and forth when the switch is
thrown. Clean the contacts and wiper with electronic tuner cleaner and make
sure the two metal points on the wiper make good contact with the plates.
If necessary, GENTLY bend the points upward to reach the contacts. Now
re-assemble and try the switch. If you are lucky you can quit now.
3. If it still doesn't work and if you can find a volt-ohmmeter, use the
ohmmeter to check the continuity through the contacts and rails to determine
where the open point in the circuit is. If you don't have a VOM, you could
try the same thing with a battery, a flashlight bulb, and a couple of
lengths of wire.
4. Only in desperation if nothing else works, I would consider cleaning the
wiper points and contact plates with extremely fine (600) wet-or-dry
sandpaper. But you risk wrecking the points. I would definitely do step #3
to determine where the problem is first.