747
you def got a lemon of a gun. Like I've posted in this thread my Shield 45 had the same problem. I have the mag catch replaced and that fixed the issue (I am fortunate to live about 1 hr from S&W).
I would hope S&W would refund the gun or the shop where it was bought. It be somewhat comforting that the Other Shield 45s you tested were problem free. Sounds like an entirely new gun would be your next course of action.
Love my shield although love my Glock 22 better