Sounds like the switch itself is probably faulty.

Easiest way to test the switch is with the Game diagnostics. 
Go to switch test and depress the switch with your finger.  If it doesn't register every time then it is most likely bad and needs to be replaced.

Greg at RTBB has the replacement part if you need it.
http://www.rtbb.com.au/catalog/product_info.php?manufacturers_id=10&products_id=1279