First make sure it really is the switch that's bad and not a problem somewhere else in the wiring. One way to do this is to clip a wire onto the terminal on the switch and the other end to ground. If the light is on solid then it probably really is the switch. If you still don't get a good solid neutral light, there is some other wiring issue.
Usual problems with the switch itself are minor wear and loss of spring tension. Take the switch off, clean the contact areas carefully, making sure to remove any dirt etc. that could get between contact areas. Again try shorting to ground directly from the inside of the switch contact to make sure it makes the light come on fully. Finally, check the contact spring to make sure it is pushing solidly on the internal contact and on the shift drum end. Somewhere in that sequence it should work again!
