On the flues, at least the field grade, the cocking lever goes through the bbl lug at an angle downard as it goes to the rear to catch under the toes of the hammers. It is held there by the forend iron, removal of the forend allowing disassembly. If the forend is not being held firmly as Jim said, it could be allowing movment of the bbls before starting to cock the gun.
Miller

Miller/TN
I Didn't Say Everything I Said, Yogi Berra