[CowTowner]

A short while ago, I built an AR type rifle in 7.62x39mm caliber.
Once assembled, the bolt did not travel all the way to the rear when fired.
I tried PMC, Privi Partizan and Wolf ammo without any change.
The next round in the mag would not feed. The last round would not lock the bolt open.
I contacted the upper supplier and was advised to send the upper and bolt carrier to them for repair. After getting it back I inquired as to the cause of the failure and was told the issue was "The carrier was fouled and the gas rings were misaligned.".
Now I'm not the brightest bulb in the pack, but I still remember how to clean and assemble the bolt carrier group on an AR type rifle. I learned in the 70's when issued an M-16A1 in the Army.
I understand what "the gas rings were misaligned" means. I'm not sure I agree with that, because I know I checked them more than twice.

So, what exactly does "The carrier was fouled" mean?

Oh yes, I was warned that available ammo was under powered and may not cycle the action correctly as well.