For some reason weak spark comes to mind. There is a lot of controversy out there about this but I have heard that if the spark is weak it will not fire under compression. The idea is that the resistance of the gap is greater under compression than in 1 atmosphere air.
I googled "weak spark under compression". It's a thing.. the forum won't let me post URL's but one that popped up with an explanation was on yesterdaystractors.com
It's a long shot, but since you have compression, gas, air, and spark (at the right time?) maybe look at the coil, wires, and/or pug.