As you've seen, primer selection and cartridge loads can effect the function of many old doubles. So can firing pin tip condition. If they've been roughed up by corrosive priming in the past, they can stick. The action's cocking speed is also a consideration. Because of the design of the Parker's action, it can pull the tumblers back to cocked position a bit slower than other designs. (For more details, please see my new book, Birth of The Boxlock) It can all add up.
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