I bought a 6.5 grendel upper from PSA and let my kids play a little this weekend. I bought a 10 round AR Stoner and a 4 round Alexander Arms E-lander. The rifle cycles perfectly with the AR Stoner until the last three rounds, and the rifle won't pick up a cartridge at all with the Alexander Arms unless I manually cycle the bcg. Does this sound like a magazine problem, or a rifle problem?

What buffer are you using in your rifle?

The rifle has a PSA lower and upper, so whatever they use. The lower worked well with a 223 upper and a 300 blackout upper.

I have an 18 inch, mid-length gas system grendel that was doing the same thing. Bolt was cycling faster than the mag spring would push the cartridge up to feed, over-gassed. Changed to an adjustable gas block, turned the gas down a bit to slow it down, and now it runs like a champ. A heavier buffer should accomplish the same thing.

My e-lander mags work fine.

Check your fired brass to see if that PSA has a rough chamber. (Any chamber marks will show in the brass, look for rings or chatter marks.)

Make sure the internals are lubed well, especially the bolt and all the lube points on the carrier. That's probably obvious to you, but I've seen several guys have problems because they took it straight out of the box and shot it without cleaning or lubing.

Other than that, it's hard to tell from your description what the rifle is doing for sure. Any pictures or more description of the issue would help.
How much of the bolt pushes against the case head when loading from that E-lander mag? Does it just barely catch, or is there plenty of engagement?

I have a PSA 18" 6.5 Grendel upper and it's fine. I believe your upper should have an adjustable gas block. I would start there, if not a heavier buffer. I have he same mags as you only 10 and 17 round mags (I think) with no issues from them.

If it ejects fine without a mag in it, it’s probably a mag issue. I’ve found mine is picky.

Load one round. So when you load the chamber you have a chambered round and an empty mag. Fire and see if the bolt locks open. Do this about 10 times.
If at any time the bolt fails to lock back the rifle is short cycling. That means it may be best to open the gas port up about .005" more.
If it fails to lock open most times you'll need to open the gas port up about .010" to .012" to start with.

The gas block adjustment did the trick. Cycles perfect now. Thanks guys.