I was also getting frustrated so I enabled the cheat menu (Camp -> Take an action) and it provides debugging info as you attempt to recruit.
What I found interesting is that in situations like many of you describe above (high relation/high honor/high renown) there is still a random roll made against a % chance of recruiting. The highest chance of recruiting I have seen yet is like 40-50%, so still only 1-in-2 chance. Often times it is much lower, like 10% chance. Sometimes it is 0% or even negative chance.
I completely wiped out Mumain, took their king hostage, had high pre-existing relations with all Mumain lords, and still could not recruit them until I took a look at cheat menu to see what was going on. Since cheat menu also allows you to reseed intrigue I decided that if a lord has greater than 40% chance I would just keep re-rolling until it succeeded. Makes no sense otherwise because you'll never get lords.
And I have like 1 city and 3 towers just waiting to hand out.
Val