The gun may be recoiling so fast that the firing pin does not have time to retract after firing. (to heavy recoil spring.) Or the firing pin is hanging up in the pin channel of the slide.
May I suggest to disassemble and clean the firing pin, spring and channel paying particular attention to the hole in the receiver where the firing pin protrudes through to ignite the primer. This pin and area can buildup carbon residual. It is not common but not unmentioned.
Do not oil only...
Some Shield slides will stay out of battery app 1/8" if the slide is not allowed to sling shot to normal closed position, do not ride the slide when releasing (chambering a round" and do not expect it to fully close when doing a press check.
Yes, the Recoil Spring is stiff as also are the Magazine Springs suggest that you check out the UpLULA. These are not firearm brand specific. Talon Rubberized Gripes are also an often added option for the Shield.
Amazon.com : UpLULA Ruger Mag Loader - 9mm to 45ACP Maglula Uplula Pistol Speed...
You may have better luck posting this question the S&W Semi Auto Forum
http://smith-wessonforum.com/smith-wesson-semi-auto-pistols/
Your post is in the M&P Forum
http://smith-wessonforum.com/smith-wesson-m-p-pistols/