Just my thoughts, has there been any case neck separation you may have part of the cartridge in the chamber, is there any crud in the extractor ramp area of the barrel.

Is the bolt head fully wound home and not out by one revolution.

You could try removing the extractor & spring to see if this has any bearing on the problem and the extractor screw in the side of the body can be too long sometimes fouling on the bolt when it is going in to the lock position, compounded by the force of the extractor spring pushing the bolt head away slightly