Personally, would clean that nipple and the vent leading from nipple to breech well. the easiest (and proably the best) way is to get a can of carb. cleaner....run a DRY patch in from the mussle to the breech...remvoe the nipple...spray the carb cleaner theought the nipple seat, through the vent, to the breech (Whre the clean dry patch will absorb it). Try to wrok a pipe cleaner through to the barrel...a lot of "bvunk" gets plated on the passageway, narrowing it, so making sure it is clean and gunk-free is the first step in getting sure-fire ignition. Also use the carb cleaner to blow out the nipple (making sure the nipple is 100% clear) and then give the gun a try.
If using pyrodex...stop and try black. Black is easier to ignite, and many of the older designs really need BP to light off as they should.
I did give up on one and install the musket cap nipple (and had to modifiy the hammer to get a good solid strike)...but ti wqas a cheap reproduction with two hard truens in the flash channel from nipple to powder....most only have one 90degree angle.
One one older pistol, installed a "flame and go" (unfortunately no longer made) "fusil" that converted to a sealed unit that took small pistol primers in order to shoot pyrodex...the pistol would work fine with BP, but pyrodex (and esp. old Pyrodex before the middle 1980's) is harder to ingnite and tended to missfire.