+1 DC. The variety makes a huge difference, and its particular placement. Some zoas like lots of light, some like it a little lower in the tank. A good way to tell is to frag it, and put one frag high, one low and see which one is doing better.
Also, feeding might help. Eric Borneman, in his book Corals, wrote that some zoas are more inclined to take food than others - some like phytoplankton rather than zooplankton. Cyclopeeze has worked for me, my corals take it greedily, and seem to be growing faster. Some of my Armor of God palys didn't start splitting until I started feeding them.