If I remember correctly, this lock uses a fly with a small hole in it--if so, try turning the fly over as sometimes that makes a difference.
Another possibility is the sear spring may not be pushing the sear into the full cock notch. I had one L&R lock with this problem, and it drove me nuts before I figured it out. I annealed the spring and opened it a little wider, then re-tempered. It's been working perfectly since.