The brass is supposed to have enough elasticity that it springs out and seals the chamber under pressure, and springs back enough to create clearance to be extracted without too much effort. At least under conditions of normal pressure. Hopefully the extraction problem disappears when you operate the gas system with every shot. I've only ever had the situation you describe, of stuck brass, after firing, and I didn't really count, 40-50 shots rapidly in succession at a pin shoot. Seemed like when the gun got hot enough, a round stuck. I'm speculating here, but maybe the pressure went too high when the chamber got really hot and heat soaked into the as yet unfired next cartridge. It's never happened to me again, but I've never put the gun through that sort of conditions since.