Hello, I'm having a bit of a problem with the second nut at the top of the strut. I haven't played around in this area for thirty years (mark 1) and wonder if I'm forgetting something

The nut turns with some resistance as though it is coming off, the shaft does not rotate but there are still seven threads showing after lots of turning. It's an ordinary nut, not the slotted variety. I've tried with the car on the ground and car raised up, bottom of the shock fixed and then unbolted. Same result.
Could the nut be stripped or is something else happening? Do I need a brain transplant?
I was feeling quite pleased with myself after my first ever glow plug change over the weekend and now feel somewhat embarrassed to be stumped by such trivia
Grateful for any (respectful

) comments or ideas!
Philip