I would get some #11 caps and try them on the nipples with the chambers unloaded. If a couple cylinders full (12 caps) fire OK then try loading one chamber and capping with a #11, then firing it. If the caps blows off the nipple it may be too large, or the charge may be too heavy. If it stays intact (or at least doesn't blow completely off) then you cantry a full cylinder.
Changing brands of caps is also a good possibility for solving the problem.