I always had good luck with the MSM spitfire nipples available from Mountain States Muzzleloading. You might also try a different brand of caps, my favorites were the CCI #11 magnums, they work well for hunting also, but not as good as a musket cap. If this is a hunting gun, that might be a consideration you should look at, replacing the nipple with a musket nipple if they are available and the hammer will fit the larger musket caps. I've been using the flangeless musket caps lately from D&R, much nicer than the hat type. It sounds like you may already be doing this, but to seat a tight cap on a nipple, lower the hammer to the cap gently, then press it into place with the hammer, not your thumb. You can put a lot more pressure on the cap that way, just press gently but firmly though! hth, Tim