I really like Fritz's idea!
In addition to that:
When I get "herbivorous" fish that I do not see eat within 24-48 hrs of going into the tank I do something horrible: I try to increase the amount of nuisance algae in the tank!
I'm not saying that I recommend you to do that, my situation calls for desperate measures (the tank is at work, so I do not have the leisure to watch a fish for hours/consistently).
I leave the tank on for longer than I would like and within 2 days I get a thin film of green on the glass (not on rocks!). When I get decent growth (when I can see it on the glass, that's enough) I reduce the light cycle for the same amount of days, then bring it back to normal (and no more algae build up).
I did this when we got a yellow tang.
She started nipping at the glass, and overtime started to go for the food in the water column. Now she nips at the Garlic Nori strips, goes after the herbivore blend with the other fish, etc.