I used to offer mentoring in a certain programming language via email. It soon became too time-intensive to continue with it, mostly because the people appeared unwilling or unable to do the necessary groundwork and just wanted answers without much effort on their part. They wanted, basically, supervision.
I recall reading a very short book years ago, I think it was "The One-Minute Manager", and the biggest takeaway I got from that is that you have to adopt different styles according to people's motivation and skill (a 2x2 grid similar to [1]).
It appears "mentoring" (coaching) is appropriate for people who have skill but lack motivation or confidence. This definitely correlates with my experiences.