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HF & the world of muzzleloaders...
Been hunting with side lock since 70's... Yet, dont claim ta know it all...
Difference in side lock & what your shooting is, if not mistaking yours is in-line, nipple is in-line with chamber, barrell, powder... The 209 is more of a shotgun primmer, ruffly twice the size of the #10 &11 caps used on side lock, & nipple is
not in-line with barrel, rather off ta side, hence the name...
Also the powder used in the in-lines is a more modern gradr of powder compaired to old black powder... Dont think your using the loose powder, rather the pellet type, they are a little tougher ta ignite if not mistaken, part of reason for the bigger primmer....
Of all the shooting i've done over the years, seldom had misfire... The powder your using is a lot cleaner more modern & should have less foul ups.. Can get roughly 2-3 shots with out running patch ta clean, 3rd shot is tough ta seat... Use dry patch if continue shooting, if wet patch, & then dry, dry fire first before loading ta make sure all dampness out... With breach plug it a lot easier... It could be in your nipple, faulty from factory, something not letting a good spark, the pellets little harder ta ignote reason in-line... Could be a bad batch of pellets, Good spark, nothing obstructing, dry chamber should ignite...
Hopefully some one will be here ta advise better, Dont give up... As pappy says:
keep your head held high & your powder dry ...
edit: make sure pellet & minni, maxi ball is seated proprrly... Put ramrod down barrel, mrk depth... line up your 2-3 pellets with maxiball & subtract that off mark on ramrod... That way when every thing seated properly, the line on ramrod be even with end off barrel...