Although it sounds like you have a restriction if the other barrel works fine, you might try using musket size nipples. The RWS caps are said to be hotter. I have two of those Pedersoli shotguns and mine work great using Remington #11 caps. CCIseems to work as well. I use Black MZ or 777 powder. I have gotten into the practice of firing off a charge of powder alone in each barrel before actually putting any shot in. Never have misfires after that. Like someone said, the powder may have absorbed too much moisture. I have had that stuff get useless on me before. Try some fresh and then dump that other stuff.