switch brands of caps.
Believe it or not they all fit differently. Remington caps fit on Remington Nipples, but you put other brands of caps on Remington nipples and they don't fit quite as well. Usually too tight for the remington nipple. So it takes one for the hammer to push it on tighter and then the second time will make it fire. I sadly had to show a Gun smith this very thing. For the 7 years or so that Remington made in-line muzzleloaders in the Reminton 700 frame, this gun smith had been modifying the nipples to make them work. Wow - he felt like an ass when I told him, here try these, and bang every time.