How about when they stumble, don't get it, do not have the experience of the wisdom?
How about when and if they lie, 'do their thing', hide?
Do you walk away and simply say they are not teachable or something similar and lay blame on the student?
Possibly interview some people you respect who are good or great mentors and ask them what some of the core values are of a mentor. I by the way don't have a magic list of core values.