My friend had a similar experience during a late season ML hunt in PA. I was doing a push for him and jumped a big doe that was headed his way. I stopped and waited, and waited, and waited but no shot. When I got up to him he told me the doe came by him not 15 yards away. He bleated and she stopped. But his hammer would not stay at full cock.
After taking my little tool kit out of my pack and removing the lock we both immediately saw the problem. When he put the lock on the stock, the rear (set) trigger was forward and the sear arm was below rather than above the trigger.