Susan, give them a few goes at the same Nori before giving up. I usually find my Yellow Tang and Coral Beauty are a but hesitant at first, but after putting in the same Nori a few times (and when they realise they aint getting anything else) they start eating it.
I have tried several different sea weeds from the Asian supermarkets. All were from China. They usually have the scientific name of the algae written on them. All have been eaten so far except for one type that came as knotted kelp - only the Strombus ate that one.
I did have one packet from David Jones that the fish really liked and was made in Japan, but David Jones doesn't stock it any more.
I currently use the Obento brand suggested by Magic (it's even cheaper at Asian Supermarkets).
Aside, I let the algae grow on the back ball and one side of my aquarium so my fish are "free range" eaters and the top half of the back wall always has the half-moon impressions from my fish eating at it.