Causes:

Dusts adhere to the switch.

Solutions:

Remove the dusts. Alternatively, put the switch in a case or use a seal switch.

Causes:

Insulating films are produced on the contact surface due to hazardous gas around the switch or switching in a low load area.

Solutions:

Replace the switch with a one containing environmentally-resistant contact materials. (gold, alloyed metal, etc.)

Causes:

Soldering flux penetrates the switch.

Solutions:

Review the soldering method and use a switch that flux does not penetrate.

Causes:

Dusts adhere to the switch.

Solutions:

Remove the dusts. Alternatively, put the switch in a case or use a seal switch.