I can't imagine its a problem with the nipple. If the cap is firing but not igniting the powder the problem is either week caps. Everyone I have ever talked to says the RWS musket caps are best and the only ones I've used. OR the problem is the powder....it is really easy to contaminate powder, it collects moisture if old, in humid environment, gets handled or touched with fingers or oils, barrel is not dry and clean, if you keep cleaning the barrel and leaving even a small amount of moisture in the barrel it can containment your powder.
If it was me I would buy RWS caps and try that, easy and cheap. Also I would think about the powder and making sure no oils or moisture was inside my barrel. But I would bet its bad caps.