How to coach this kid?

I am a new coach in my first year as a linemen coach with a local YFL team. Typically, I do not believe in yelling and screaming but do elevate my voice until I can gain players attention, and find that is enough to put things back in order. However, one young man on the team, whom is by far one of the most valuable members of the team, often becomes disconnected and unattached with any attempts to coach of correct his behavior. Its as if he simply shuts down. The other coaches see it as a lack of effort or attitude problem, but I am not so sure. It is as if after noticing praise or doing well, the kid tries to fail. If he is not doing it wrong, to him things don't seem right. Am I just being soft, or do you think the kid needs to be coached differently?