I bought a near new 336 made in 1969 that had lived in the JM vault until the Remington takeover around 2010(?). No ignition problems at all.
So unless the hammer spring seems abnormally light, indicating that a previous owner has shortened it too much, that is not the problem.
One test that you could try is to cycle the action and then try pushing the firing pin forward with your fore finger. Because of the safety spring acting on the rear section of the firing pin,
this will normally require about a pound or so of pressure. If it is significantly more difficult to push forward then the firing pin channel is gummed up or a burr exists on the firing pin.
Also possible is that the firing pin is broken or that a burr exists on the breech bolt where the rear part of the firing pin presses against the forward portion.
Last edited by saddlegun; 04/02/21.