I have the exact same problem. Clan level 4 a lot of influence and 2 castles 2 cities.We conquered half the map and the settlements always go to the same 4 clans...
Is there some kind of official info anywhere how the candidates are determined?
So far I only found speculation regarding relationship scores with the orher clans, proximity to your existing fiefs and so forth...
„God damnit Dethert I‘m your son in law, gifted you 6 grandsons and -daughters.
My (MY!!!!) elite warriors win your battles, not the other loosers with their armies of recruits! Yet you give all settlements to them. FU dude, FU!“