with your finder. Then too, if I am not too presumptious, or disrespectful, are you scanning both sides of the stud and fining that mark then scanning the other side of that stud and then marking that point. this should give you the outer boundaries on both sides, therefore the center should be in the center of that. I had an apprentice that had a very hard time with a sensitive finder, he would drill at the first beep which would usually land him too far over, but when he did it this other way, he found every one.
If this isnot the case, then do the 3d finish nail exploratory operation to find one, then you can find them all.