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.