I would take the lock out again. Loosen the screws and wiggle everything around to make sure everything is in the right position. Tighten the screws again and operate the lock by hand to see if it appears to be working properly. While the lock is out, inspect the inside of the mortise on the stock to see if the end of the arm of the sear is touching the wood. Also, inspect the mortise well to see if there appears to be any rubbing between the lock works and the wood. Anything dragging inside there will weaken the force the set trigger applies when it hits the sear arm. Also, when you re-install the lock, back the lock bolt off about 1/2 or 3/4 turn after you screw it down tight.