Bolt lug over case jam, so bolt is not cycling far enough back to get behind the case rim to drive it out of the mag cleanly.
So either not enough port gas to cycle the B/C all the way back (bolt unlocking too late).
Or, too much port gas with the spent case still over pressurized to the chamber walls from the bore residual pressure not bleed down yet from the bolt unlocking too soon to cause the short stroking instead.
As for factory loads, should not be so long that the rounds are binding up in the mags from to back, to cause a feeding problem. Also, since other ammo's are running fine in the rig and mag, its the ammo that is causing the short stroking in the rig from either bolt unlocking to slow, or too fast. Hence the reason that the spent case rim will tell the tale of what is going on with the bolt unlock timing.