mdfoy -
Almost all materials expand and contract with changes in temperature and humidity. The recess for the binding post plate is a snug fit. Over time, it can become even more snug, which is you are experiencing.
Here is what we would do. Fashion a stick that is 3/4" x 3/4" x about 16 - 18" long. Remove the woofer and run the stick through the woofer hole to the rear of the binding post plate. While someone is holding the plate and terminals at the rear of the speaker, use a hammer to tap on the stick and drive the plate loose. Increase the tap pressure until the binding post plate comes loose. It eventually will.
This is the technique we use when faced with this situation.
- Jim