Do you put the nori on a clip and leave it in the tank for awhile? Sometimes tangs don't go after nori like other foods, but they usually will graze on it if its in the tank.
Also, soaking it in a vitamin solution will greatly improve the likelihood that the tang will eat it and that it will benefit him.
Another thing to try would be some Spirulina flake, which you can usually get at a LFS -- its good for herbivores and very nutritious.
FWIW, you may have gotten a tang that was roughly handled during capture and shipping -- could have been doomed from the start.
Good luck.
Mariner