Chasing a misfire.

There are only a few possibilities. Here's all I can think of;
1) the lifter; is pumped up, or the seat is stuck up, or it is not on the base-circle
2) the pushrod; is too long, or is not seated properly, or has the wrong ends on it
3) the rocker arms are defective
4) there is a problem at the valve to seat interface , like a slightly bent valve
5) the cam is broken?

Of these, the most likely is #1, which @Killer6 already mentioned.
I would start comparing parts with another cylinder, with an eye to swapping them around.
You can back the rocker arms right off on the offending cylinder, and then do a LD test. The air pressure will push the piston to the bottom so wait for it to go. then read the leakage and determine where it is going. You don't even need a fancy tool for this, just inject compressed air and listen. Anything between 60psi and 120 is good but shut the compressor off so you can hear the leak.