i would say your sear is dragging or making complete contact once you tighten the lock down .
some places to look .
A) lock inlet between sear pivot and sear hole
B) back , bottom edge of sear hole
C) inside bottom of the sear hole . IE the end of the sear arm is bottoming out
D) contact with the top of the trigger bar
it actually doesn’t take much to block the sear from engaging the full cock . there is normally only about 1/32 drop in the sear between the 1/2 and full . so any contact can cause a problem .
IE if the lock inlet is not level , it will allow the lock to tip slightly as you tighten it down.
i would use a little inletting black or just a black marker and coat the sear . put the lock back in and tighten it down . work the lock , then take the lock out . the place that rubs will then have a black mark . simply and gently take off the wood that is marked and then repeat tell the lock works properly .
Now another place to look that I have seen cause a problem , is the main spring not having enough clearance when the lock is tightened down . IE it contact either the edge of the barrel or wood around the lock bolt and or barrel area . Again because the lock inlet isn’t level and when tightened down , the lock tilts just enough . Again it doesn’t take much .
As to not having enough wood ??
You should be able to remove the troublesome areas as they are inside the inlet.
The two areas that can cause issues with thin stocks would be B,C above .
With B) you can take your lock out and bring it to full cock . Note that the sear will drop to very near the bottom edge of the lock . So your sear hole must also be even with the bottom edge of the lock .
With C) if the gun is real narrow , then you may not be able to deepen the sear hole . In which case you simply remove a little off the end of the sear bar . It actually only need to be long enough to cross the trigger bar .
Ill send you a PM with my # if you need me to walk you through it over the phone , i can do that