It takes a while for some of them to get used to it, but keep putting it in and eventually they will take to it. I have five Tangs and they are currently in three different Tanks until they are all done with QT so I can add them all at once. My Powder Blue took forever to actually start eating the Nori, but my Purple Tang was eating like a pig from day 1. My Blue Hippo and Two Spot Bristletooth Tangs both pick at it all day, but seem to prefer Mysis Shrimp and pellet food for now. Finally, my Achilles Tang is starting to pick at the Nori on the clip, but will totally devour the Hikari Seaweed Extreme pellets that I bought as an alternative to the Nori sheets. I assume once they all go in the same tank it will pick the habbit up from watching the others so I am not concerned at this point.
On a side note: my One Spot Foxface absolutely loves the Nori and goes through the entire half sheet I put in his tank alone. He does seem to like the red seaweed better for some reason so I have 2 clips in my DT and once all my fish are in it I will be loading both clips with different colors to give them options.