Did you check the solder/connection to the coil that powers VUK? Since the coil does not fire all the time in test mode thats where the problem is most likely originating from. You could have a connection issue somewhere along the coil line. It would be a good idea to make reseat the connection on the board that goes to that coil.

The switch has absolutely nothing to do with the kicker not firing sometimes in coil test mode. Having extra switch or two never hurts though so not discouraging you from getting it just trying to address the issue.

Now if your doing switch test by rolling ball in to trigger it and it doesn't fire then it may very well be the switch. But if in coil test and it doesn't fire all the time it's not a switch issue. If you haven't already test coil in coil test mode to see if it fires all the time. Hope that make sense.