Problem with first round loading

[Blowby]

Sounds like the BCG is not returning properly with enough force with the most resistance of 20 rounds in that type of magazine. Check for any burr's or wear marks to show drag? Also make sure the alignment of the receiver extension tube to the rear threaded lower matches the upper's bolt path. To test this lock the bolt back and very slowly release it to see if any drag occurs. You might notice the BCG releases slow or hear a little drag. You have to release it so slow like you are trying to make it stick. This will happen at the rear most position.

I assume it's lubed properly but just wanted to throw that out. Maybe it just needs a squirt of Amsoil.