A weak hammer spring or excess headspace could both cause your problem as noted by posters above.
In addition, it's possible your firing pin is too short. The firing pin can be damaged by hot loads. This can happen in one of two ways. A pierced primer will let hot gas back that acts like a blow torch on the firing pin thus shortening it. A hot load can also cause the action to spring a little trapping the firing pin in the primer crater and making the action difficult to open. When one uses sufficient force to open the action, a part of the tip of the firing pin is sheared off.
The firing pin is easy to replace in the Encore (how do I know?). There is an Allen headed screw under the hammer that needs to be removed. Since there isn't much room to get an Allen wrench in there, it's easiest to grind one down to the appropriate size. Once that's loosened, you can easily remove the firing pin, spring, etc.
Hard primers could also cause an ignition problem. Unless you're using military spec primers, this shouldn't happen. Actually, it shouldn't happen even with military spec primers but it may anyway.
