Headspace is checked without the extractor or ejector.
This is because the ejectors spring action particularly gets in the way and requires force for chambering, which is confusing. You can do a quick field check with those parts installed just isn't a great check.
Your problem isn't headspace, is likely timing or just some little machining defect. I don't know about lantac or rosco, no idea if they are top quality or bottom of the barrel, but if it was say PSA I would be blaming it on cheap parts and saying you should determine if its bolt or barrel and getting a replacement (which sounds a lot like barrel).
One knowledgeable person told me they grease the lugs and manually cycle any new barrel or bolt 200 times, ie charge it and let it slam shut, repeat. It is broken in enough to shoot when the weight of the BCG alone closes the bolt smoothly sliding down the upper at a 45 degree angle, else more dry cycling or new parts.