umm well this has never worked for me but what probably affects it is how long there at war with you and the relationship with the king and if it was a normal game how bad they are being beaten would affect if they want peace but that doesn't seem to be true.If you want a better relation ship with the kings you need to be capable to beat there forces if and when the option to capture the king comes up let him go and your relationship with him will go up and you'll also get some honer.
Edit: the only time emissary works for me is when im fighting agents vagirs