The misfires with my M-29 always perplexed me. It is just fine now, but I should clarify what would happen... I'd shoot single action. I pull on the trigger would sometimes not fire, but the let-off would be abnormal--IOW the hammer would drop before the trigger was pulled to it's normal "let off" postion...the round would not fire and the firing pin/hammer would not make contact with the primer. Never happened with my other 3 S & W revolvers--I believe the M-29 is a M29-5...
Off hand I haven't a clue as to what might cause this.
When you cock the hammer to shoot SA, the hammer foot forces the trigger back. In turn, the trigger strut forces the rebound slide aft, and the pin in the rebound slide cams the hammer block down (so as to allow the nose of the hammer to pass over it, into the firing position).
I could hypothesize some failure of the pin on the rebound slide that, coupled with binding of the hammer block in its raceway in the side plate (I have
seen hammer blocks deformed when a user tries to remount the side plate while the hammer block is not properly lined up with its raceway), might allow the hammer block to hang in the "up" position notwithstanding having cocked the hammer. However, I don't really see how lubrication might fix this problem.
You might try a couple of things. One: after insuring that the revolver is unloaded, insert a fired
case (with spent primer intact) into one charge hole; slip a piece of thin paper behind the fired case, close the cylinder and align for next shot; cock the hammer and pull the trigger; examine the paper to see if the pin touched the spent primer. Two: invert an unloaded revolver and cock it, then examine the hammer tunnel (you'll probably need a mirror and bright light) to see if the hammer block is in the nominal "down" (firing) position.
The reason I say to invert the revolver is that is is at least theoretically possible that the rebound slide pin is missing; the hammer block bound in the up (non-firing) position; and when you lubed it you released the binding so that the hammer block was dropped into the down (firing) position and resides there full time. The fact of the matter is that I'm speculating, perhaps unrealistically; as noted, trained eyes should look at this revolver.
The bottom line, though, is that this revolver needs the attention of a qualified S&W smith, even though it appears to be functioning properly now.