do you have a fuge and if so what do you have in it?? naso and several of the other tangs
are finicky at best although i did find a way to get them to eat and im not saying this will work with every tang but it might help.
when i had my 150rr set up i had a powder blue and some yellow and a blue or purple tang
everyone of them ate like the food was going extinct.sometimes theres something in the water that they are looking for and your not giving it to them. i have found that if you give them a little of the macro algae that you keep in the sump they tend to tear it up.
something like letting a child smell freshly baked cookies and then telling them they need to eat liver and onions first.there not gonna eat the liver and onions but if you give them alittle cookie first the liver dosent taste so bad.
also sit and watch him for an hour straight he may be eating and you dont know it.
tangs are grazers theyll pick a little here and a little there i once had a fish that never ate what i fed the tank. i had him for over two years. came home one day and he was belly up. its just how it works with fish. sometimes there is just no right answer.
anyway hope that helps.