It may be something very simple but most likely you have already checked. In the past I had a front action screw that when properly torqued it barely rubbed against the bolt head. You couldn't feel it on an empty chamber but had noticeable drag with a case in the chamber.
I headspace all mine on the tight side of go-gauge and factory brass (saami) is always loose.
Might be worth looking at that screw if you haven't already. I color it with a Sharpie on the side of the bolt head.
Hope you get it figured out.
Randy