Have you tried a new nipple? When you said cap I am guessing your shooting a #11 cap. Perhaps the longer cap, so the bolt can close harder in the shorter distance.

I own a rifle with the same problems you describe. Its a Knight LK rifle. I know this sounds crazy, and I have done it on the range but I have taken an empty rifle. The one with a problem. And capped it then fired it. If the cap did not fire, I would then load it, and hunt with it. It then is sitting on the second firing.