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