Sooner or later you have a fly rod ferrule that refuses to come apart. Here’s a neat little trick that usually saves the day. You’ll need a piece of aluminum foil; I used discarded scrap. And you’ll need a handful of ice cubes.
Place the ferrule on a table. Slide the foil underneath the rod. Load up the foil with ice cubes. Be sure to place some ice under the ferrule as well on top. Roll the foil up like a football, so the ice is tight to the rod. Wait at least four minutes. Open the foil. Now try pulling the ferrule apart. It should come. If not, try the ice again for a longer period. No foil? A towel might work just as well.
PS I have not tried this trick on a metal ferrule, but it may work there too. Good luck!