I think I would be for sending it back and let NEF fix it right this time.
If that's not an option, be advised the breach pressure is probably too high making it hard to open after being shot. You'll probably have to load it down somewhat. Of course, if that's what you wanted, lower velocity, you probably would have gotten a .223, right?
The Winchester brass is probably stretching and not allowing it to rechamber.
The fliers may be coming from inconsistent lock up, which may be the cause of the brass stretching problem too.
Just my opinion, take it for what it's worth.