Pull the bolt and see if a round slides up under the extractor or if you really have to force it. Does it feed with three? Does it feed from both sides of the rail, except with 4? Might just be the mag spring is not pushing the back end of the follower up.
Those ideas seem sensible, especially checking for rim clearance between the extractor and boltface.
I’m not a Swede guy; does some brass have thicker rims than others? There’s some nugget in my poor old brain about dimensional issues with it, but I can’t locate it. Maybe another brand of brass will work better.
