I have a non PC Shield in 9mm. Given I live in CA, PC has a double meaning.

The slide "hanging up" before going into battery when new is a common problem. I too had it but only when press checking or if I "rode the slide" when chambering. It worked itself out after a few hundred rounds.

I would agree with the others. Give it a good, thorough cleaning with a proper solvent (I do this will all my new handguns), then a proper lubing with a good gun oil/grease. I also disassemble and clean all my magazines.

I am a bit confused by your comment "it would lock back further than the slide lock catches even". You mean it locks back all the way to the rear?

Was UMC the only ammo brand you shot? I'll assume it was 115gr. I've haven't shot any UMC in my Shield. I haven't had issues with UMC in my other handguns but have had intermittent problems with Winchester White Box (as an example). Point being that sometimes these things can be ammo related.

Keep us posted on your progress.

Welcome to the forum and congrats on acquiring an outstanding firearm despite the issues you've encountered.