Since this is in the NFA section, is it safe to assume you're running a suppressor?
300BLK barrels are usually advertised that they won't properly cycle with subsonic ammo without a can. I've got 9" and 16" uppers in that caliber and run both with an H2 buffer. Supers cycle 100% but the subs really only work 100% when running suppressed.
The beauty of that round is that when you suppress the subsonic round using the same gas system you get very little back in your face due to over-gassing. If they won't cycle WITH a can, that's a totally different beast.