The bolt may be misfitted. Try prying the leg that rides the hammer's cam out a bit from the other, into a slight "Y" so it hugs the hammer harder, it should be a spring pressure fit between the frame wall on one side and the hammer on the other. Don't overbend, it can break. If the legs are weak or cracked, there won't be enough pressure to stay atop the cam. Slowly cock the gun, if the bolt drops too soon (before the half-cock notch, it may be the above problem, or you may need a new bolt, it may have had the legs cut too short. Also inspect the hammer's cam that it is not rounded at the top edge or otherwise misshaped and dropping the bolt early.