There is a fair amount of randomness to recruiting lords. I think the highest chance I've seen (using debug mode) is about 75%. I'm fairly certain it's impossible to get a 100% success chance. I always save before talking to a lord and reload because I'm cheap like that lol.
Also all of my infantryman are nords If i have one tiny little skirmish with any nord lord a handful of my best troops always defect even though I had 99 morale beforehand.
If you're at war with the Nords your Nord troops will have negative morale based on how low your relation is with the Nords. However I'm pretty sure it shouldn't be enough for them to leave your party if your normal morale is 99. I only know is that they will get a penalty to morale based on your relation with the Kingdom of the Nords.