ry the new #10 Remington caps. They have improved there caps two fold and they are about the best I have tried. They are also much hotter. You don't say how your loading the caps?? If you are useing your finger you need to run that cap on down on the nipple with a doll rod so that it bottoms out on the nipple.Make sure that the nipples are clean and dry. I like to use pipe cleaners in the hole and then a pick. Then I hold the cylinder up to the light and look through the chambers and the nipple holes. This is a much cheaper way to do it then fireing caps to clean and dry the nipples, that can get expensive!
Also tap the side on the cylinder each time you place a powder charge in it to settle the powder on down in the chamber closer to the nipple hole.
If none of the above works then I would think that you need new nipples or a heavier hammer spring.