there can be considerable variation between different brands of caps. CCI#11 gave unreliable ignition on my Ruger, CCI#10 was even worse, Remington #11 gave several misfires with Pyrodex, only more-or-less satisfactory ones were RWS#1075. I made a pusher-on tool out of brass rod, counterbored to accept the cap, and turned to clear the contour of the back of the frame. Now it never (seldom) misfires.
You might try another brand of replacement nipples
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