There are people who believe in the monopoly of Knowledge. They feel that the skills they possess should be exclusively theirs even when they are obliged to impact the knowledge to someone else. They are reluctant to teach people may be for the fear of losing relevance. Or the fear of the learner out shining them.
How else can one explain why a student doing a work practice is told that a particular job is too difficult to teach? What is the student doing at the work place is not to learn new things, including the most difficult? This is the lamest excuse a professional could give for not guiding a motivated student new skills.
This scenario is an example of people who are afraid of other people’s success. They think that the success of others diminishes their relevance in that particular profession. I think this is not the case. The world is big enough to contain more skillful and successful workers. The more they are, the better for everyone.
My advice is never hoard your skills when you have an opportunity to teach it to another person, especially, when you’re obliged to do so. This is because you learn new things each time you teach someone else.