So its hard to say with limited information. I would be curious on flow, par, all parameters....... However if everything else is doing great, I would inspect for pests. There are many pests that go after zoas...
Not sure that would be the issue. What other coral do you have? Any other Zoas that are not having an issue? Any algae growing on them? It would be great to know all your parameters as well. As of now I would lean toward light, flow or pests....