Hmmm.... No sparks hitting in the pan, hang fires,,, and all this in high humidity/rain...
I'm sure your groups advice is sound, they've been shooting in these conditions and recommend the caplock in these current conditions. Now in a controlled area (like the other day when you got 6 good pan flashes - no moister conditions), I just kinda think it's "moister" related. I like the idea of 3fg powder.
Now,,, could the frizzen need re'hardened? Possibly... Drilling out the vent hole to 5/64ths (most certainly I would)... Giving the face of the vent hole a small bevel, absolutely...
Will a new replacement lock solve all the above problems??? Maybe, maybe not??? Will re'hardening the current Lock's Frizzen help??? Again, maybe - maybe not...
I would suggest (but it's entirely up to you),,, when the opportunity presents itself to get your rifle out to the Range in "not so humid" conditions, take advantage of it and see if after enlarging the vent hole and beveling the face of the vent hole a tad - see if it helps...
Now, one last thought. How much priming powder are you using? (Full Pan, 2/3rds Pan, 1/2 Pan, 1/3rd Pan, a combination of all?) I've got locks that work great with very little prime, and others that require 2/3rds Pan of prime. Just have to find what works best for your flintlock ignition.
Flintlocks are finicky in wet weather, there's just no two ways about it. Your best bet is to keep that vent chamber as dry as possible and free of oil and moister. Same goes for the pan. One thing you can do is plug your vent hole before and after loading - then pull that vent plug (feathers work great) - pick the vent hole - wipe the pan - prime - and then aim and fire... Work a pipe cleaner into the vent after firing - wipe the pan - then repeat the loading and firing process.
Again, I do think your results would be better in dryer conditions, but for now it's all guess work and don't feel you're all by yourself with this current problem, we who shoot flintlocks have all been there at one time or another.