I have had the same situation occur to me, bit with a Romy AK. It was with Winchester white box ammo, and like in your case, the bullet lodged just at the rocking, not allowing another round to chamber. I really cannot not determine an exact cause, bit I do speculate that it was either A) a bad load with not enough charge, or B) an incorrectly sized bullet, or one that was defective. I am still not sure to this day, and I did not keep the bullet for a closer examination. So I have no real evidence to go on. I just chalked it up to one that got by QC.
Oh, I forgot to add, that the AK did extract the shell, and the following round would not chamber. So I guess it may have been a improper diameter projectile, as opposed to not enough charge. But that would lead me to believe that there should have been some signs of damage, or stress on the rifle of it took the full brunt of a charge at the trunnion and the bullet never left the chamber. It would seem like it would have blown up in my face, but it did not, obviously, and it did not destroy the girls, as it runs like a Timex to this day. Very odd, and fortunate on us both it seems.