Had this happen to a saw I just picked up, it would give me an indication on the neon tester but not with a spark plug. I checked the kill switch by disconnecting it, no luck there. Then I checked the coil gap, seemed excessive. I adjusted it down to where it should be with a shim and got a nice fat spark. So the tester was actually indicating a weak spark where the plug would give you none at all, in this way the tester actually helped with the diagnostic.