you'll find a side lock much easier to get into. you don;t need to remove the stock to get at the workings
there is a bolt/pin next to the left hammer that goes through and comes out in the same spot on the right side plate
you should be able to remove both locks once this is out. with the lock out, you can see if dirt or wood splinters getting in the way. Clean the lock's insides with a toothbrush (use an old one or your wife's - solvents taste terrible
)
now you can hold the lock in hand and cock the hammer to see if the nose of the sear catches in the tumbler's notch. Just keep a firm grip on the hammer when you need to trip the sear's extension (sits at a right angle to the lock - enters a hole in the stock to meet the triggers). you do not want this dry firing.
If a good cleaning resolves the issue - great news. other wise you need to decide if the issue is the sear or tumbler. taking a lock apart is not that hard once you have done it a few times- its a bit daunting the first time. But you could just send the lock to someone like Brian a lot easier than sending an 8 bore.
If the lock works fine out of the gun, then look to the trigger, is it not properly resetting in place and keeping the sear from coming all the way down. I actually had one that this was all it was. took the triggers out, cleaned them , took out a slight bend, and cleaned the slots in the stock they go in and everything was good to go.
let us know how it goes.
one hint- when I take a lock apart- i draw the plate on a piece of paper. then tape each piece and screw on that paper with an arrow drawn to the spot on the outline of the plate to remember where it sits. I have taken many locks apart. but it still do this- I don't lose small parts and have fewer DOH moments putting it back together. and watch the small parts, some try to escape. this is a job for a clean workspace. I do it on a large white towel.