id do the following
might not help but doesn't cost nuffin
clean the ceramic part of each plug in brake cleaner or even dish soap and water
squeaky clean.. if the electricity can track across the ceramic in the dirt you get no spark
clean out the connectors on the ends on the wires in a similar way. dirt in the rubber boot can do the same thing.. well it can in damp wet places like where i'm from
make sure the screw on parts on the plug tops are tight if they need to stay in place for your leads.
try a spare or different set of leads...
if that doesn't help this does cost a bit
i have one of these (sadly sold out on your local website, but decent info)
Gunson Colortune See Through Spark Plug Kit G4074 G4171 G4172
its a see-thru spark plug with a little telescope and mirror so you can see the colour of the flame in the chamber..
dismissed by many as a waste of time. but hey there is no convincing some
diagnosing distribution issues in a multi branch manifold, setting idle mixture and just off idle running mixture... really quite handy, 1960s tech. but hey it works
Aim for a blue-ish flame and the car runs great. make sure you get the right size plug/adapters
works wonders if you have a carter BBD with two mixture screws on a six cylinder
the two screws have more or less of an impact on different cylinders depending on the branching/shape of the manifold.
special plug in each cylinder and work out which screw has most influence on it and note it down
use that to help navigate the rich at the middle cylinders lean at 1 and 6 situation that you can occasionally see on a six. basically get 2 and 5 right with both screws at the same setting
tweak for a nice no 3/4 and no 1/6 based on the notes you took.
for this issue I'd run it in the one that does not fire and take a look. No flames to see? use the long plug wire adapter and you can basically use any conductor from the distributor cap to the adapter as a temporary plug wire to discount the normal plug wire. just don't electrocute yourself.
if you can see a flame, the colour of it will dictate if its way too rich or way too lean to fire consistently...too rich will eventually end up in a wet plug, no spark situation the plug just shorts out.
Dave
