There is no apparent logic behind what prompts a lord to come over, or refuse.
Recently, I'm sort of coming to the conclusion that the "punitive" amount of collateral the lords demand as condition for coming over, is actually an in-game "inhibition" set for both the player and the AI, to simply BLOCK people from taking over an entire kingdom by simply save-scumming persuasion checks to all lords during peace time.
Judging by the amount even millions of gold fill on the persuasion bar (which doesn't even fill 20%), the amount of money the lords are demanding from you ranges over 20~30 millions of gold. This is much too high a sum to be considered a normal part of a normal game, and therefore, the only possible conclusion on WHY this is so, is that it's INTENTIONALLY SET TO IMPOSSIBLE LEVELS TO ACT AS AN INHIBITOR TO STOP THE PLAYER from persuading everyone to join player faction.
If the above assumption is true, then it explains why some lords, those with even towns and castles, will come over to your side upon successful persuasion, along with all their land and directly adding to the player's strength.
My theory, is that through some parameter set at some phase of the game, basically two or three lords in a given faction are set as "can be persuaded." The rest of them, are just hard-set to always ask for such amounts of money under whatever circumstances, and some of them are set to come over to your side, but leave their lands behind.
Through a lot of experimentation, empirically, it seems like, (for example) if a certain faction has 20 clans total, maybe 2 of them will come over with their own land almost unconditionally. Around 3 of them will come over, but leave their lands behind. And the rest 15 will be simply "impossible to persuade." My theory is, the ratio of how many clans will be under which type, are just simply set from the start. So far, I've never met a situation where a lord that has land, that demanded impossible amounts of money, would ever change that attitude.
The only times where their attitude changes, is when they've lost their fief, and are landless. In this case, they often become easy to persuade. This, is also another circumstantial evidence that lords refusing to leave their own faction to join yours, is fundamentally tied with whether they own their fiefs or not -- which supports the above conclusion that the punitive money demanded, is an artificial "BLOCKER" mechanism set to stop the player from taking over huge amounts of land through persuasion alone.
....
If the above theories are true, then I can certainly understand and accept why the game is set up this way, but I must also add this is a very crude method of handling the situation. A very poor execution of the intent -- as certainly there are better methods to handle it.
Since all of this is (probably) a matter of programming and event triggers, I'm also guessing it might be a crude, place-holder mechanic before they implement something better, which I certainly hope is the case.
