What's the malfunction? I have one in .40 and had a few feed failures. It appears it was just a bad box of ammo. I've got about 10000 rounds through it since then without a hiccup.
I had a few bad feeds when I first got it... Pretty sure it was operator error though. Think I accidentally hit the mag release button a few times... it's a lot closer than I'm used to. Have to be careful about where I keep my thumbs!
OK, so you may have a mechanical problem but I would shoot the hell out of it for well over 1000 rounds and see what self defense ammo works best with no malfunctions. I have well over 2000 K rounds through my Shield with not one malfunction. Keep shooting and if the number of failures stays call Smith and Wesson and explain your problem I am sure the will fix it.