Its not the humidity, we have that up here in NC as well, and my flinters go off with no problem...Its probably crud in your drum...Try loading with powder (no ball) 20 grains is enough and then remove the nipple, it you don't see powder under the nipple, the powder can't ignite...Clean the area where the drum goes into the barrel, a pipe cleaner should do it.. .While you have your nipple out, see if you can look through it, if not clean or replace...I have replaced the drum on several muzzleloaders, these were screw in type drums, not sure what yours is....But, when I replace a drum I always cut a taper inside the drum so the powder can flow under the nipple easier...
When you clean your caplock, remove the nipple and thoroughly flush out the barrel with either hot, soapy water or rubbing alcohol...By removing the nipple you aren't leaving all that crud in the drum area...then dry and lube and you should be good to go...
Good Luck