From what you have said, It may be a few things. Firstly, have you checked the non-firing cylinders spark plug against an earth outside the cylinder, to make sure the plug is actually firing? the other two things I can think of are a bent valve, which would reduce compression considerably depending on how bent it was, or a piston ring that has broken down?? A compression tester is onlyabout £20 from halfords, well worth it for diagnostic work. The only other thing I can think of is to check the ignition system is not earthing anywhere else due to a worn HT lead or cracked coil etc?