Is your boat trailer single or duals? With the hires trailer, Do you think any wheel sets (particularly the rears) came off the deck when you reversed up.
Could you imagine the trailer sitting on the front set only, with the rears clear of the deck, putting extra upward pressure on the hitch?
There are a few particular leverage combinations which could come into effect at any given time.
Yes, there is some adjustment available via the screw nut on top.
From another forum.
Adjusting 50mm Ball Coupling:The coupling should have been supplied correctly adjusted.
If however someone has been playing with it or it needs adjustment for some other reason it's a simple job.
1. Remove the ball from the tow bar.
2. Undo lock nut on coupling adjusting stud.
3. Place ball in coupling.
4. Whilst jiggling the ball slowly and gently tighten the adjusting stud with a large screw driver until you feel all slack disappear from the ball.
5. Back off adjusting stud 1/2 a turn.
6. While holding the stud in position with the screw driver, tighten the lock
No responsibility taken in my behalf, but this seems about right IMO.