Finally got to shoot my latest build this weekend and I had a couple of issues.

I built this gun myself.  I've had the lower forever and never had a problem with it on the previous (Stag) upper.  The new upper is as follows:

Colt upper receiver with M4 cuts
Armalite mid-length barrel with M4 cuts
LMT semi BCG with BCM extractor upgrade
Low-pro gas block (drilled and pinned in place by ADCO)
Carbine buffer

Mags: 6 30-round C-Products stainless with Magpul followers and CS springs
Ammo: Remington UMC .223

I was shooting off-hand or off a bench and went through about 100 rounds total.  Accuracy was fine, and ejection seemed good.  Out and back five or six feet.

Before the FTF, I had problems with the bolt locking back on the last shot.  Some mags never locked back, some almost always locked back, but none did so 100% of the time.

The FTF pictured below happened twice, both times with the same mag.  I couldn't reliably reproduce it with that mag; it just happened the two times.  Both times occurred in the middle of a string when the mag was loaded with between five and 10 rounds.  I was shooting probably one round per second.

My first impression is that the bolt isn't making it all the way back before returning forward, but that's just a guess.  I want to say that I've heard the UMC .223 is a little light.   I also just remembered while typing this that I may have a Wolff extra power action spring, but I'm not entirely sure.  The jam only let the bolt go about halfway forward, but it was easy enough to pull the charging handle back.

The top case in the below photos is a live round, not spent brass.  The last picture is one of those.  It's got two big dents near the neck and a scrape back toward the base that isn't visible.  

Your guess sounds pretty good. Not familiar with the spring you spoke of but if it's a buffer spring put a "stock" one in and see if it helps. Also may want to try a heavier Buffer. FWIW

Buffer springs do wear out, and magazines can be bad.

I never had a hiccup with these same mags on my previous upper.  I'm not clear on why the buffer spring wearing out would cause this.  It seems like the opposite, that the spring is maybe too strong and not allowing the carrier to travel back far enough.  Same thing with the buffer.  Not saying that can't be it, just that if it is, I don't understand why it would be the case.

I appreciate that sometimes mags just suck and that sometimes one part just doesn't want to work with another, but I'd like to determine exactly what is making this upper appear less reliable than the old one.  Ideally, I'd like to correct the problem, but I'd at least like to identify it.

Again, I think it's either the buffer spring or buffer itself. Not that it's too weak... that wasn't my response... That it's too strong or you need a heavier buffer. I really think a heavier buffer may cure it. Are you running full-auto?

Sorry, I was responding to the above poster's comment about springs wearing out.  

If the problem is that the carrier isn't going back far enough, wouldn't a heavier buffer just make that worse?  If there's not enough juice to move the existing one back, it seems like it would move a heavier one back even less.  

No full auto, not even particularly fast semi.  In trying to recreate the jam, I did fire about as fast as I could, but didn't get the problem again.

I may get corrected here but I think the initial "whack" the buffer takes will get it going, the extra weight will keep it going. The reason I think this is because a friend of mine has an M16 that was doing the same thing. He had replaced the original stock with a collapsable stock that already had a buffer and buffer-spring. Turns out the buffer was from an AR15 and was too light. We put a heavier buffer in and it fixed the problem.

I would install standard extractor spring because of underpowered load extractor doesn't engage the case properly.

Mid-leght gas system already softens the return of the bolt carrier. H car buffer may help too