I have a 11.5" bcm barrel build on a W.A.R. receiver that I finally got to give a try today.

It initially seemed to run okay, maybe it missed picking up the next round once in a mag, but after 30 rounds it stopped picking up the next cartridge entirely.
It is ejecting the spent casing fine, it just doesn't want to pick up the next one.

I did have the gas in the normal setting on the receiver, even turned it to supressed setting and then it failed to extract the spent casing which is to be expected.

I have a H buffer in there now, I am kicking myself for not trying another mag, but this colt 20 rounder has been fine in everything else so I didn't think of ruling that out.
Although racking the charging handle it always picked up the next round just fine.

Any ideas on what I  should be looking at?

Thanks!

Sounds like it is short stroking
The simplest, easiest test would be to put in a carbine buffer, on unsuppressed mode, and shoot 5.56 ammo. If it runs fine, you have your diagnosis.
If not, try a different mag

Sounds like something is up with that bcg. Id probably start with rings... check for staggered gaps, missing ring, and overall feel. check out the gas key. Make sure its tight and not leaking. Check the key opening for size if you can.

Make sure the firing pin retainer is all the way in.chech for buts developing on the bolt lugs or cam pin.

Something is either dragging or gas is escaping.