They (Clans)consider several things in order of importance:
1: Their own fiefs and finances, no fief = easy to get, less money they have, the easier too. Clans with a fief are very expensive (like 1m-2m probably 5m+ now) sometimes and only bring thier fief if you're at war with thier faction
2: Power situation of their faction versus yours. When their faction is ****'d and you're is okay, they are easier to get. The power is the field parties + garrisons (not militia), the more fiefs and clans (parties) your faction has the more attractive you are!
3: Their relation to thier ruler and thier relation to you. If they like their king they are hard (expensive) to get and more likely to flip flop back to that faction later, but if they don't like them but have a good relation to you, they are easier and will stay.
When a lord flat out refuses you it means the hypothetical amount of money they would want to turn is just too great compared to what you posses, the devs did this so you don't waste your time on the persuasion dialogue only to not be able to get them anyways.
SO go after those poor home less clans (make some too by taking thier fiefs!) , defeat them and release after battle for +relations, hire some merc clans and you'll be getting vassals in not time (maybe...)! The more you have the easier it is to get more because you power goes up!
Just to be clear, you cannot compensate for them having a fief or you faction being much weaker then theirs's just by having high relations. A lot of people assume so, but it isn't so. It will help for sure but they need to be poor, scared and preferably mad at king too!
Further more some changes to the importance of thier relation to ruler may have been added at some point, I feel it might matter more then in older versions.
Also, I feel that having more clans in your faction may prevent (or lower) lords from wanting to join you and prompt the "I'm happy with my liege". To me it seemed once I had a good amount of clans (like 20) other clans would never want to join even if all favorable condition were met
Also related, the ability to make new clans from wanderers is very powerful and would I suggest that after passing loyalty/security policies, you use all your influence to make clans for the rest of the game whenever possible.
