It may be locking back but is it locking on the bolt face (correct) or the bolt carrier (wrong)?
Are there one or two rubber rings around the extractor spring?
The Winchester 147g white box is also a little underpowered, just like the XM80C. May want to take that into consideration, and compare those to other ammo to see if it's making a difference.
What length buffer assembly are you using? I need someone local to test our .308 carbine length heavy buffer....