I have been having a lot of failures to feed with my AR build. It is a Spikes Tactical mid-length upper on a Seekins Precision lower with a Palmetto State Armory lower parts kit. I also have a blue Springco buffer spring and a H2 buffer in it as well. The rifle functions fine with brass cased ammo and pmags, but when I use USGI steel mags and/or steel cased ammo, I constantly get failures to feed. I have tried using multiple steel mags, but keep getting the same issue. I tried switching the upper onto my Stag arms lower, and it functioned just fine. But when I put the stag arms upper onto the Seekins lower I built, it was having the same issue. So I have concluded it is not a problem with the upper short stroking, but a problem with the lower. Any suggestions or thoughts? I know I can run just pmags and be fine, but I like the option of using USGI steel mags like all my other ARs.

I had a similar issue. A buddy has a slow mo camera and shot some video. I denied short stroking too and learned quickly that it was short stroking. I've personally never had an issue with a lower, but BCG's, gas keys, bolts and anything gas related. If that's how you are shooting it, it doesn't have near enough oil to suit my rifles...

Make sure the mag catch is installed properly with the end of the threads flush with the outside of the mag button.  New uppers typically don't like steel ammo. Try them again after a few hundred brass rounds.