Was shooting my CQBR clone today, when I noticed the first PMAG M2 with Ranger Tab failed to lock on last round. I thought it was strange, so I tried another round, and the same thing happened. All 3 I brought did the same thing when the weight was on the bottom of the floorplate. Is this common with PMAG's, or do I have other issues? They worked just fine when there was no weight on the mags. I've got several PMAG's and a few aluminum magazines, but this is the first failure of this sort, at least for this rifle. My other rifle has no issues of this sort. The CQBR has a PSA lower, while the other rifle is an LMT lower.
Edit: Unsure if this unrelated, but ejection was around 4-5 o'clock, but functioned fine otherwise.
Edit #2 Ammo was Federal AE XM193.

What's a ranger tab?

Guess the correct term is Ranger Plate, shown here.

Something unrelated to the topic.

TMender, that's very weird bro. I wouldn't think the ranger plate could affect this. Functionally on the inside of the mag, the ranger plate is the same as the stock floorplate.  It's only on the outside of the mag that it's any different. I'm thinking your problem is either unrelated to the ranger plates, or maybe you found a way to re-assemble the mag incorrectly?

I'm leaning towards whole different issue tho. Post pics of the mags (first how they are now, then taken apart)

I was suspecting short stroking, as the ejection chart says, but after removing the rail and checking the gas block all is well. The bolt catch is working on the magazines, tilting doesn't matter, still holds open. I'm wondering since the gas rings are fairly new, as is the rifle (~180 rounds) since I've built it, it needs broken in more. Probably could have used more lube, as it was really dry when l disassembled it. It's sat for 6 months while I was deployed. The odd part was that the magazines only didn't lock back when resting the magazine on the floorplate (semi monopod since the rifle was on bags).

Update: I noticed two magazines both dated 3/13, had some excess flash inside the body in the rear where the follower was catching at the top. Was very subtle, but maybe just enough to cause it to fail. The irony is that these are the first time I've used these magazines, I bought them brand new about 2 years ago, loaded them and didn't touch them until last week. More testing the next time I go out will determine if that was the issue.

Keep me updated bro. It'll get sorted out. I've also had to dremel down flashing in a couple pmags, I then put a LIGHT ass coating of CLP over the ground spot, and they ran like intended afterwards.

I just found it strange. I thought it was isolated to one M2 PMAG until the other one did it, the M3 I brought did just fine, and looking at it, it's design is different in that location, and a non window model. The 3rd M2 I brought worked fine, but I wasn't bearing down on it like the other two either.

So to revisit this, took one of the magazines that I had known to cause failure to hold the bolt open out again today, and again using the magazine as a rest (on my leg) it failed to hold the bolt open. Is this common to happen, or are my PMAG's defective/weapon issue?

I can get this to duplicate repeatedly with M2 magazines, M3, and USGI work just fine. Not sure what's different. My other rifle has no problems. It seems as if the follower is stuck until weight is taken off the bottom of the magazine, the the bolt catch will rise. Is this one of the reasons why Magpul made the M3? I'm wondering if it's "over inserted"?

Is the mag catch all the way in with the mag inserted? (Left side of receiver)

If not its overinserted. If the catch is fully engaged in the mags relief cut then its a different issue. Wish I could see it in person.

I can see the catch touching the follower, in fact it holds it down until weight is removed, then the catch comes up with the follower.

Well, fixed the issue. As seen in the below photo, the bolt catch's leading edge was hitting the back of the magazine. M3's and USGI don't have material in this location. I ended up filing down the leading edge until the magazine cleared. My guess is that the channel that holds the bolt catch was machined just a hair further forward, or the channel not quite wide enough. Either way all good now, just strange tolerance stacking.