The glass is slipping, sliding around, and the strange thing is it seems to be getting smaller & smaller till it will eventually fall out alltogether.
Thinking of crazy-gluing it to the edges but it goes too much to one edge more - like the complete outer edge is being ground down.
How or why did this happen? It's a good Schneider Kreuznach.
If it is a Schneider Kreuznach POL, it is then a B+W, and
most unlikely to have undersized, loose or ill-fitting glass. SK/B+W would be very keen to learn about a filter behaving this way, and go out of their way to look at it, probably offering a warranty. Assuming it is a new filter?
In my kit all of the POL filters are SK/B+W, some many years old with no deterioration of glass, fit or movement.
It isn't a counterfeit B+W is it?
Older filters and those by Tokina/HOYA and Tiffen, use a circular spring retainer to anchor the glass in place; indeed, many filters from Japan have this; it is a tight fit but one which can, over a very long period of time, become lose. The SK filters are precision assembled; got one in front of me and there is no way the glass can be made to move, but then it's a Kaesemann polariser — nothing is intended to move other than the outer ring for polarisation!
