A headspace issue as others have already stated.

Segregate the non-firing rounds and measure the headspace with a comparator. Compare measurements with fired brass. I believe you will find that the non-fired rounds have a headspace DL which is too short.

You should only be bumping your shoulder about .001" to .002" from fired brass in a bolt rifle. AR rounds should be bumped .003"- .005" and that is why the rounds will fire in your AR.