Are you sure the bolt is completely forward and in battery.On the rounds that did not fire?
Did you tap it with your hand each time before firring? Of course not that you should have to.But if having a problem like that i would do that on each round to make sure shes locked.Hows the gas system condition? What setting is it running on.Is it completely clean and very well lubed? And the flutes are spot less? The chamber is smooth and no marks from broken shells being removed in the past? And if so emery cloth and polish it up !