What brand of #11 caps do you use? I generally use CCI # 11 caps, and have had no problems to date. But, some rifles do seem to offer more reliable ignition with Remington caps. It is possible yours may be one of them. Also, have you checked your nipple's flash hole? If you are using the rifle's original nipple, the flash hole may have enlarged, over time/use. So the flame is no longer a concentrated jet, into the main charge. Another option: swap out your current percussion nipple, with one that uses musket caps. There is a nipple replacement available for percussion rifles like yours. Its called the 'Mag Spark'. Made by Warren Custom Outdoor in Warren, OH, USA. These nipples are designed to use any 209 shotshell primer. If interested, the website is: warrencustomoutdoor.com. E-mail:
phil@warrencustomoutdoor.com. Ph: 330-307-0166. May be worth a look.
There is another product called: Accra Shot primer adapter. This one took a small rifle primer. Have seen this item listed on e-bay.
Another cause might be your hammer spring. it may have weakened over time, so that the hammer isn't falling with enough force to reliably ignite the caps. Lastly: are the caps seating fully onto the nipple? This can cause unreliable ignition. Many percussion revolver shooters will often use a small dowel, or wooden clothes pin, to gently seat the cap fully onto the nipple. I do this with percussion rifles, too. Reliable ignition, every time. Caution: Don't seat caps with your thumb! Ask me how I know this is a bad idea!
Hope some of this is useful.