I've had this problem too. However, in my case, it was not due to the switch but to the plastic part that presses on the switch. It had worn down over time and no longer had enough profile to trigger the switch. I solved the problem by sanding it down with a precision grinder and then gluing on a small piece of plastic (I based the dimensions on those of the click that was still working). It's working like new again :) P.S. for the plastic source, I simply used a zip tie.