Grab an old business card and clean the contacts

Then just depress the switch contacts to make sure they complete the circuit and fire the coil. (Close the front door if applicable as it switches off the power to the coils when open)

If it fires the coil - then you need to adjust with a pair of pointy nose pliers, moving the lower blade closer to the upper blade. Do this with the machine off. You should see a similar distance between the contacts as with the other bumpers.

Sometimes, just cleaning the points with a business card can fix the issue.

You can buy contact adjustment tools - Try Greg's ;

http://www.rtbb.com.au/catalog/product_info.php?cPath=127_153&products_id=1243

This is what I use. It is the correct tool for the job.