I had a ZKW465 in .22 Hornet that did this exact thing. It's a serious pain in the arse isn't it? I found that every factory load I tried ejected fine, but of every reloaded round one in two or three stuck. Dimesionally, the Lee .22 Hornet full length sizing die was within spec but the taper on the factory chamber was on the smaller side of tolerance being Euro rather than US.
I suspect that you may have a similar thing going on, and you may need to run a shellholder across a sander to take a few thou off and get a bit more crunch on the shells. Your idea of having a really good measure up is the right one, and also take to a few with a whiteboard marker and run lines up the cases to try and see where they are grabbing the chamber wall. In effect, primary extraction in conjunction with the grip of the extractor is not enough to pop the fired case free - but if you are saying a light pop from a rod the extractor won't ever pull that free...
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