My problems come from in the field. I use to take my AR's out in the freezing Canadian cold, leave them sit for awhile after shooting and then recommenced afterwards and I've had FTE, FTF consistently from almost every single one. And I clean my rifles every 100rds and sometimes more or less but always around there. I have dragged these guns in the dirt, in the snow, in the mud. But I have to admit, I have always been able to get the gun running in the field with no cleaning kit and just a bayonet or pocket too. I am by no means in the military and I use to shoot 5rd mags in my rifles. I've only owned Armalites, Colts, and 1 ZM. Out of all of them the ZM is the one I liked the most and preformed in a manner I expect a Military rifle.
Maybe if I lived in a region of the US where 1 inch of snow means the kids stay home, and that If I never took my rifle off the comfort of the range I would know of these invincible AR's.
And yes I have tried green followers, different mag springs, whole different uppers, different lubricants for all kinds of weather, expensive to cheap ammo, etc etc. The next AR I buy will be a ZM LR300, and that's if I ever buy one again.