Sounds like the same problem I had.
I'd clean and lube the Shield and head to the range.
I might get through one magazine,
maybe two, then a jam, then maybe another trouble free magazine before it happened again.
And they were the drop the mag, rack the slide,
insert the mag, rack the slide to fire type of jams. Bad, bad.
After 3 trips to the range and 150-200 rounds,
I felt the same way thinking, "how can I ever trust this gun?"
I sent S&W CS pictures of the extractor because I was convinced that was the culprit.
They emailed back and said the extractor looked normal but I should send the gun in for a look see.
What S&W did was give the gun a very aggressive polishing of the breech face and chamber.
Meaning it needed more than a casual chamber polish.
I didn't dare go that far, even though I am trained machinist/toolmaker, I'm still capable of screwing things up
and voiding warranties.
S&W has the tools and know how to check tolerances
and do what they have to do to get it up & running.
Even if it involves replacing bad out of spec parts.
Haven't had a hiccup since and have shot many boxes of ammo, including the dreaded Winchester White Box and other cheapie brands (perfecta, fed aluminum case etc.).
Talk about renewed confidence.
Good Luck! And let us know how it comes out when you get it back.
just sent my shield 9 mm back.
same failure to extract issue.
about 3-5 percent.
not happy at all and do not trust it.
Hope S&W has a fix for this.
it seems to be a common problem