Perhaps you don’t have your mental image of him/her worked out well enough? (Or, perhaps you and (s)he have a different idea of the way (s)he wants to look.) In either case, one thing you can do is spend some time with that character during waking hours. Think about what he looks like, and make sure you have a clear mental image of him. One thing I do that works well is meditate on your characters. That gives you a conscious world you can control and enter. From there, you could talk to that character face to face and work the problems out at the source, so to speak. (The trouble approaching him could be a number of things, but I’d do that first and see if that doesn’t fix it.)