Hi Mike,
I am not sure if you have 18mm plugs or the normal 14mm, but my compression gauge has both thread sizes, so check that out. also, if you have not done a comp check before, make sure the throttle is wide open when doing this test, with all plugs removed.
Popping in the air filter if due to a warped head and blowing gasket, is one cylinder firing through to its neighbour when it has an open inlet valve.
regards, Richard