This happened to me quite frequently when I first started shooting my renegade. Every time it happened I had to pull the nipple and put some powder under it to get it to fire off the main charge. I would get about the same number of shots before it started happening, about 6-8 shots. It was even worse on rainy days with a lot of moisture in the air.
I was swabbing every shot with a damp patch and a factory sized jag, which was too tight of a fit so it was pushing all the fouling down the barrel and into the flame channel. After reading on here a bit I came across the suggestion to turn down the jag a bit. I chucked it into my cordless and then held it against a flat file, checked fit with patch in the barrel, turned it down a bit more, etc., etc., until it fit "right". You want the jag and cleaning patch to go down the barrel fairly easily and then have the patch bunch up when you pull it back out so that it brings the fouling out with it instead of pushing it down the bore.
I very rarely have a fail to fire now during a range session.