A friend has a GPR that does not fire reliably. He is experienced. It used to work fine. I suspect the passage ways in the plug are caked up with fouling. The obvious thing is to remove the plug and see.
I have a real hydraulic barrel vice and proper wrenches. Some modern rifles, TC, CVA and invest Arms are designed to make it near impossible to take the plug out and not wreck anything.
Are the Lyman GPR's a problem also?