I have a cap and ball pistol that did that I found a weak spring in the fram that broke when I pulled it out I replaced it with one I ordered from Dixi gun works cost me about 6 bucks as I ordered four of them just in case one of my other revolvers does the same thing ..... but weak or broken flat springs is a common thing with a well used pistol kinda like anything that is enjoyed it will break down in time so take it apart if you can and check the springs other wise a local gun smith can do it for you at a cost .. keep - em - shooting and enjoyI have a black powder pistol and when I pull back the hammer to try to engage it, it will not lock into place. Does anyone know what can cause this?