Urchins, foxface, tangs not eating diatoms: normal?

[Miami Reef]

My list of herbivores:
Yellow tang
Naso tang
Orange shoulder tang
Blue tang
Foxface
2 pincushion urchins
1 tuxedo urchin

It seems like only the orange shoulder tang and the tuxedo urchin are eating the diatoms. Is this normal? I expected all these herbivores to go to town on the diatoms.

 

[blaxsun]

Yes. I would try some ring cowries to help with the diatoms.