Went back out today, ran 3 mags of s&b and slide locked back just fine. On the 4th, no lock back. Brought some Blazer with me also so loaded up a mag of those and it locked back just fine. Finished off the s&b, using one round of the Blazer as my last round and the slide locked back just fine. Looks like it's the s&b doing it. I only have a couple boxes left of those, so feel better.