How long have you had the yellow tang? Sometimes they don't like to come out in plain view while eating. Maybe if you rubber-band it to a small piece of coral and sink it to the back of the tank where he can eat it while he hides, he might be more tempted. If he is a new arrival to your tank, it may take him a little while to start eating. Also offer some meaty foods, such as brine or mysis shrimp, or some krill; they love that! Best of luck!