I agree.
zoas come from different ocean, and need different nutrition, water conditions etc. it takes them a while to adjust. As long has you have a healthy tank, leave them alone, give them good lighting and flow, they should start to multiply.
sometimes a tank condition is right for one type of zoa, but not for another, so you end up with "slow growers"
some of my zoas need extremely strong lighting, and temp less than 80, but others would burn in strong lighting and thrive in warm waters...
Zoas also go through growth spurts, sometimes they'd grow extremely fast, and then stops for a while.
best advise: patience