Bill Tindall, E.Tn.
I had this problem on a mower that had a fine thread for the blade bolt.....what were they thinking. I solved it with a slather of anti-seize compound on the threads. This stuff is a suspension of what appears to be metal and molybdenum sulfide in grease.
What is seizing could be either the washer surface or I think more likely the threads. What is the thread pattern(acme?), size and pitch?