I am not an expert on single trigger guns but I have worked on several. Some things that come to mind is that you may want to make sure the gun is cocked then take off the side plates (especially the one that doubles) and check the catch to make sure the slot that holds the hammer back is not worn or has a chip of wood hung in it. It that seems ok you may want to play with screw tighting. Tightness of the screws can mess with a single trigger. Also with one sideplate on and one off you can look through the single trigger to see how much clearance you have from the trigger mechanism to the lever that releases the hammer. I have had to grind some off the bottom of the lever to give it more clearance. Best of all is to take it to someone that is good with single triggers.