[Old and Reliable]

Ian- I have a set (RH and LH) of hammers with intact pins from a later 12 Parker Trojan that may work, if needed here- also the coil springs and retainers- Dean is 100% right, 9 times out of 10, crud and dried oil are often the culprit-

Here's how I check for firing pin depth and primer pocket penetration- with Parker assembled and hammers cocked, remove the forearm and barrels, and hold a piece of 3/4" or 1" clean and knot free soft pine against the standing breech face- release safety and fire each hammer with striker into the wood, then check both holes for uniform penetration with a pin depth gauge. If that looks OK, it might be shells with extra hard primers such as B&P or Fiocchi-

Sometimes a check of the firing pin holes is warranted- gas blowback can often enlarge and make them "out of round"- and excessive extractor drag or scuff marks may indicate a slight "off face" condition, easily corrected here as we have many top gunsmiths in America- not the case where you are I surmise however.

Another possibility- perhaps just the first trigger (right hand barrel) is mis-firing, often that is the most fired of a double, but that is just a supposition- with the gun re-assembled and the hammers recocked, remove the forearm and try this- try to 'wiggle" the barrels to see for off face or excessive play- and then with the forearm still off, mount the gun as if you were going to shoot it, and bump the stock into your shoulder pocket to see if there is any movement of the barrels against the frame.

It might help us help you if you could detail and the ID numbers found on your Parker- barrel flats and lug (frame size), receiver, forearm iron.

What game do you get to hunt with this Parker in your country??