Having seen this exact problem before, My quess is that what you need to do is open the top lever bounce the action on your knee to open it. The problem I have encountered is with non-rebounding locks, the firing pin is pushed into the "primer" of the snap caps, and won't let the gun open as the pin is protruding. What I had to do was break the edge around the "primer" in the snap cap. Actually, this is a problem with a lot of snap caps and some are so modified.
Not your gunsmith's fault, fault of the snap caps.
Again IMHO
