Doubletrouble,
on a double trigger gun, about the only thing that could mechanically be wrong is a tired sear or maybe a tired sear spring. If the spring is weak, it may not be putting enough dowward pressure on the sear when cocking to fully engage the sear on the hammer.
If the gun was restocked, it could be that not enough wood was removed in a particular spot and causing the problem.
Never say never, but... IF the trigger plate was warped, it may or may not cause this issue since the sears and all other related cocking parts are attached to the frame, not the trigger plate. But the triggers are on the plate and they could be contacting the sears a bit if the plate was warped. Who knows.
I hope I was able to lend a few ideas as to the root of the issue.
