I learned this tip from pal Brad Bachelder- I had the same issue with my 12 gauge Parker GHE project gun, and also my late Dad's 12 gauge M21- sometimes a burr or accumulated crud causes the ejector stem to "grab" the non-fired ejector stem, when the breech is opened and the ejector hammer is tripped. Back out the stop set machine screw from the base of the barrel lump, with the barrels and forearm removed, and on a padded bench-- then slide out carefully each ejector stem, and guide pins, if so equipped. If they seem "stuck" I squirt some Kroil into the hole for the stop screw and let the barrel stand for 24 hrs-- then tap the back edge of the ejector "lugger" with a 1/4" soft wood dowell and a small plastic hammer, until each ejector blade and stem comes free from the breech. Then clean out with a .22 bronze brush and Hope's the "tunnel" that the ejector stems rest in place, and with OO steel wool, clean with Hoppe's each ejector, completely. Then check the inside mating "flats" of each ejector stem, for burrs and crud-
What may be happening is that a burr "carries" the other ejector blade out when it is tripped- removing those burrs with a honing stone and fine steel wool usually cures the problem. Use a toothpick carefully with Hoppe's to clean out the guide pin holes in the breech- be careful NOT to break off the toothpick and leave a residue in the guide pin hole- re-assemble, spray with Rem Oil, and re-set the barrels and ejector forearm in place, and test with snap caps first, then with once fired plastic hulls appropriate to your gun's gauge. Snap caps, especially A-zooms, are a heavier "ejecta" than a fired plastic hull-
This procedure worked 100% on both the two shotguns I mentioned. As I enjoy working on and doing proper maintenence and cleaning/care of all my firearms, I have also done this on my graded ejector L.C. Smiths, and my Utica Fox 20 Sterlingworth ejector gun as well. A cleaned, burr and debris free action, properly lubricated, on any firearm is a good thing/RWTF
