BCG on left was failing to lock back on empty mags, and now won't at all, even when manually racking the action with an empty mag properly seated.

In the exact same gun with the exact same mags, the right BCG always locks back on empty.

So I'm assuming there's a defect on the left BCG.  Can you see it?  If not, where should I be looking?

I would check the just the lower (remove upper) with an empty mag to be sure the BHO is being engaged properly, the magazine follower should push it all the way up with ease. Once you've verified that the BHO is working right, I'd start looking at symptoms. Is the rifle undergassed or over buffered? Has it worked reliably before? Is the dirty bolt clogged with carbon and causing a gas restriction? Can you lock the bolt in question to the rear manually? If not, why? Is something obstructing the BHO? Might need some strange acrobatics to get eyes in the ejection port while holding charging handle to the rear and pushing on the BHO. Rifle specs? Round count? Last cleaning?

This has nothing to do with anything but the BHO mechanism: I'm testing by manually cycling using the charging handle.  There is no shooting or ammo involved in this problem.

And it involves the BCG, since it fails all the time when I use the one on the left, and not at all with the one on the right.

So what has the manufacturer had to say about it?

I assembled this myself, and I don't remember where the BCG in question came from.

Can you manually lock the bolt in question to the rear? Not letting an empty mag do it, but taking your hand, and pressing the bottom of the BHO while manually holding the bolt to the rear.

How many rounds through the BCG? It looks nice and worn, are the gas rings shot? If they aren't putting enough tension on the bolt within the carrier, I can imagine during the BCGs rearward travel that the bolt could be sliding or hanging against the chamber, causing it to move forward in the carrier enough that it never clears the BHO, so the BHO never engages it. Still, to eliminate everything else, I would function check the BHO to ensure it is working right and that it is NOT in any way causing problems. Same goes for checking mags to ensure they raise the BHO when empty.

I just relooked at your picture, is the bolt fully retracted into the carrier in both images? The bolt on the left (problem bolt), looks to be rotated 10-15 degrees to the left (forward movement of bolt within BCG).

Good eye: You led me to the source of the problem!  For the photo (to get more in focus) I had pushed the bolts to the rear.  As you noticed, the problematic BCG wasn't quite making a full rotation.  As a result, the bottom lug must have been just barely dragging to hold the bolt catch out of the way or something.

When I removed the bolt I found a bunch of copper specks around the bearing ring that rides in the front section of the bolt carrier.  (I had been running this in a brand new 300BLK barrel with a ridiculously large gas port that must not have been fully deburred and so was shaving bullet jackets and sending fragments back with the gas.)

Suffice it to say: After cleaning the copper debris off the bolt body, that BCG worked properly again!

Thank you for the help!