Rezca
The Greatest Jaggi
Flexible difficulty is only rechecked every 50 turns or so (configurable in the BUG panel). You may have set min/max difficulties that prevented rebalancing too.
They've always been at -1 for the ranking part (Difficulty won't change if...) since I never understood what those meant, and the difficulty themselves was set to Warlord for minimum, and Monarch for maximum with a 50 turn inc, but even after 50 turns it never went past Noble. After a while I just changed it myself then disabled FB for the rest of the game so it wouldn't lower it, but by the time I did it hardly mattered anymore - none of the AIs ever recovered from the beatings they took.
Future games FB seemed to work much better, not really sure what went wrong with that one.
Mercy Rule is also a way to end the game if you had 'Mastery' victory enabled and ended up wiping out the world and don't have Barb Civs or Revolutions enabled