Zoas are an odd critter. I've had two of the exact same zoa side by side, where one colony is happy and the other wasn't. Differences in flow are extremely important with zoas, and any other type of soft coral IMO
So, to keep zoas happy and healthy, the answer is simple. Have proper lighting above the tank. If they don't open up, try a different spot with a different flow pattern. When they are happy they will spread. This can vary from zoa to zoa. A poor way of looking at it is that the more expensive or rare the zoa, the slower it will grow. Though a poor way of looking at it, it always seems to hold water when I look at it.