If the lock works fine out of the gun, you are most likely having a clearance issue with wood. Make sure you have adequate clearance around all moving parts. Coat everything (lock internals, triggers..etc) with candle soot, re-install, and run though the motions. My guess it that there is not enough clearance in the sear hole. When the trigger bar pushes against the sear, it might not pushing it as far as it needs to, to clear the fly and half cock. Either that, or there is issues with clearance around the fly and it is rubbing against the wood, which is not allowing it to swing freely.
Simple fix if you are able to spot the problem areas.