Hmm, run up against another unpersuadable character, one of those in Arx. It seems the first persuasion check fails regardless but the second passes regardless, so I'm not really sure what's the deal with that. I'm stuck at 6 points of persuasion (5 base, which is the max: I did try respeccing my extra point into that but found it won't let me) and to date have only found amulets with +1 persuasion so I can't raise it higher than that unless there's a potion, food, drink or a spell that I've missed.
The outcome isn't terrible with one fail and one succeed but I'm still not happy with it so would like to figure out how to increase it further... unless it's just scripted to fail, which is possible, I guess.