Almost certainly a cold-solder joint.
basically, when the switch is being soldered to the pcb, the solder did not meld to the contacts completely, so over time, the impact from you fingers will losen the connection, eventually leading to the intermittent behavior you're experiencing.
You have to take it apart and touch up that joint..
buy one of those $5 soldering irons online.. that's all you need..
you don't even need to buy any solder.