Does the match form a good 'head' on it whilst burning? Can you post a picture?
Do you blow on the head PRIOR to loading the match? And after loading the match into the serpentine, do you (with the pan cover CLOSED!) blow ACROSS the breech again to put a good head on the burning cord before firing?
TIP - Without a loaded arm or burning cord, figure out best placement where the cord needs to be positioned to hit the center of the pan bottom. Now, close the pan cover ... and SLOWLY lower the cord to just barely touch the top of the pan cover. I bet you dollars to donuts that is NOT the center of the pan cover, as whereas the pan cover is higher than the bottom of the pan, it is likely a little more forward of the center of the pan bottom (due to the arc of the serpentine). Make note of that position ... and for matchlock newbies, I suggest putting a dot of Sharpie marker in that spot as a
learning tool.
Now go load your arm, prime your arm, and light your cord. You know to BLOW OFF any stray priming powder away from the pan I hope, after you close the pan? When you go to load the matchcord, then lower it to the pan cover (that is CLOSED!) and ensure the glowing tip hits your marker spot. With time and shots down range, you'll position it best without any marker to identify the best cord position, it'll become 2nd nature ... provided you shoot often.
Otherwise, I betcha that cord you have is 'old' and has lost its
ooommmmpphhhhh ... note use of the period correct technical term there
.