TOZ-SxS firing fault

Does anyone have a cause for my TOZ-66, SxS, 12guage, 2/2/3 shell, 20" barrel, at times to misfire on the left side.when fired second. The right barrel fires fine. When I fire the left barrel first, it fires. I have placed an object between the trigger and the pin, and have indentations on point of impact, (trigger-upon exposed pin).
Further, When I placed a piece of paper between the breech and firing-pin positions, I get a strong almost penetrating impact indentation from both barrel on the shell heads. So, what is causing the left side to not fire, at times ,when fired second, after the right side is fired?
THanks. This TOZ-66 is a 30 to 40 year old Russian made SxS,Coach, 20" barrels, from the Tula Arsnal.