In the spirit of closure on this thread, my gun came back from S&W with two new magazines. They found nothing wrong with the gun itself. The new magazines function more reliably than my original ones, but I still experienced a last round issue at the range with one of them. I'm convinced there is either a design flaw in the EZ magazine or the materials used to make up the magazine components are somehow deficient. Wish S&W would fix this problem...