I want to know the causes and solutions of contact failure.
Dusts adhere to the switch.
Remove the dusts. Alternatively, put the switch in a case or use a seal switch.
Insulating films are produced on the contact surface due to hazardous gas around the switch or switching in a low load area.
Replace the switch with a one containing environmentally-resistant contact materials. (gold, alloyed metal, etc.)
Soldering flux penetrates the switch.
Review the soldering method and use a switch that flux does not penetrate.