If all else fails remove ur buffer spring and replace it with a colt spring. I had a carbine spring that was 11in. That is in spec but it was too stiff. I bought a dd spring and a bcm spring and my gun cycles tulammo now reliably. I had the same problem the bcg wouldn't retract far enough to pick the next round from the mag. Don't over look a cheap spring it might look good but it can be bad. It happened to me. I'll never buy nothing from delton again.