Ok, thank you for all of your suggestions. I guess a little more info is warranted here. The bullets are Black Bullets International coated lead bullets (black color, so the sharpie wouldn't work) rather than plated or jacketed, so if the coating is compromised during pulling, it's just about ruined. The brass was purchased from Palmetto State Armory fully processed and ready for loading and they claim that it is once fired range brass from a police range (I think). I've had this brass for a long time, so I don't remember all of the details. Now, I don't know how many cops use .380 or 9mm Makarov or .22 TCM or even .38 Super because I've found quite a bit of that brass mixed in (especially those damn .380), but I know I'm not buying that brass from PSA again. All loading has been done on a Dillon 550B with Dillon dies.
One of these rounds slipped through my QC last time and it was so fat, it completely locked up my gun. The slide was out of battery and wouldn't move forward OR backward. I tapped the muzzle with a rubber mallet and it went into battery, but no matter how strongly (and carefully because the gun was now loaded) I tried to rack the slide rearward, it wouldn't budge. I didn't know what the problem was and I was afraid the barrel link had broken somehow (STI Marauder) and had mechanically locked the gun that I didn't want to shoot it to clear it, so I had to pay a gunsmith $38 to fix it for me. I don't want to repeat that situation.
Like I originally said, it's 15 rounds out of 600, so it's not very common (2.5%). I like the idea of checking by measuring, but it sounds like a lot of work (I know I sound really lazy now). Y'all are right though, I DO need to find out why they won't load. I guess I'll save them for a rainy day to diagnose because it's too darn pretty outside to be stuck inside doing that. Thank you all for your suggestions again.