A flat cam lobe, broken push rod depending if it's exhaust, or intake, will still show compression, but won't fire. I had one that broke a pushrod that showed compression, but wouldn't fire. It's been 40 years ago, I believe it was an exhaust pushrod. Replaced pushrod, and all was good.
Run it with the valvecover off to check cam lobes, of course a pushrod should be obvious, the rocker arm was off on mine indicating a broken pushrod. Mine had screw in studs, but your heads I believe are press in studs, might be a stud coming out too.
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Brad Yost
1973 T/A (SOLD)
2005 GTO
1984 Grand Prix
100% Pontiacs in my driveway!!! What's in your driveway?
If you don't take some of the RACETRACK home with you, Ya got cheated 
  
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