There should be two springs, for the hammer and trigger. In most revolvers, the hammer will directly push on the hammer when doing double action, and it will also be the part which hooks onto it to keep it in place. There should be a little rod that connects to the trigger that pushes the cylinder around. The trigger needs to have contact with both of these parts to operate it. You should take off the side panel, and pull on the trigger, with the cylinder out, and see what the trigger is pushing on, then you could see what exactly is pushing on what in the particular gun, and what may be missing. Look if there is any little lips or anything that seem like they should catch something and aren't.