What kind of tang? The bristletooth varieties have a harder time tearing at nori with their special teeth. And some tangs eat more plankton than they do algae.
I just trained a scopas to eat nori after a couple months of struggle. If there's another tang in the tank to teach them, they tend to catch on quickly, but my scopas is the only tang in my softie tank and it took him a minute. For a fish that is used to finding algae on the rocks, it can be confusing for them to think about going towards the surface to find food.
Tying the nori to a rock and sinking it can help to train them. I'd stick with it. Roll the nori up like a cigar so it doesn't disintegrate as quickly. The fish will figure it out eventually. In the meantime, maybe try Formula Two flakes and frozen. Good luck!