I had a similar issue. PMC .223 ran fine in my Windham Weaponry SRC, but I had problems with it in my Daniel Defense M4. I ended up sending the DD upper back to them for work for fix a suspected gas block leak. But even after that with Federal 5.56 I was still having FTF's with some of my GI mags. The Troy Battlemags and Pmag's seemed to work just fine. I ditched all of my GI mags, and now use only the Troy's or Pmag's. The one Tapco mag seems fine, too.
I would try using new Pmag's and don't overload them. I think I only load mine with 25 rounds. If you still get FTF's, then there's most likely an issue with your rifle. Either it has a gas leak, has some cycling restriction (Froglube it?), or it just doesn't like PMC ammo. Or maybe you just got a weird batch of PMC. I've had rather good luck with it myself.
