Having compression cause spark loss is a mystery as to the mechanism. So moving from compression caused by a mechanical problem let's move on to an electrical problem. I don't know what kind of load a spark plug firing under compression vs ambient air has on the "impedance" the plug offers to the ignition system. I know plugs are harder to fire meaning that it takes more spark voltage amplitude to jump the same gap as compared to ambient.
With both plugs under pressure, as Star suggested, you are loading your spark producing circuitry. What would be the component that would be the most sensitive to higher voltage requirements????????? Probably the coil as Star recommended. A lot more things in there that could be sensitive to higher voltage than the solid state circuitry driving it.
I guess you have checked your wiring for breakdowns.........before you jump on the coil, wait till after dark and in a dark room, garage, shop, whatever, with both plugs installed, visually go over every inch of the wiring from the plugs, starting with the insulators to the HV leads to the plugs, to the coil, around the points where the spark plug wires exit, to the input voltage connection to the coil and back to the trigger circuit.
Look for a blue arc where there shouldn't be one. I have seen arcs down the plug insulator, arcing from the high voltage plug lead to something that is grounded or tied to 12v, arcing from the plug wire entry point in the coil to the 12v input terminal through the coil insulation, and same with the coil ground connection.
If that all looks good then if I were spending my money, since you have already changed the plugs and have had the same result, I too would change the coil. Yes you are throwing money at the problem, but having a career in the business of high voltage and it's breakdowns, it can be a sly fox to capture and right now nothing else seems to make sense.
