If this is an early #11 or Musket cap ignition, then it probably has a load of crud in the bolt that slows the firing pin travel. If you have not taken the bolt apart and cleaned it, you should look there first. I have a friend who has let a few deer get away because he sprays his bolt with some form of lubricant, but does not take it apart and clean it thoroughly. Usually the gun will fire after the first trigger pull breaks the crud loose, but the deer is gone by then.
