I had that problem once. If you want to recruit lords from Rhodoks, their kingdom must be in bad condition.Being defeated in a war for example.If there is not a condition like that , they will choose their king not you.
I was a rebel of Khergits and my relationship with lords was ok , however , they couldnt give up from their oath and my rebellion fell. Then i became a Khergit lord and started a war against Sarranid.Then i quit from khergits - my relationship with each lord was at least 25 and it didnt change after i quit- and they lost the war. This time i wasnt a rebel i started my own kingdom and nearly every single one of vassals of Khergits joined me.This is my story
