Like Fishgeek said lots of flow and good light. Not to mention prestine water quality IE 0-5 trates, 0 trites, 0 ammonia, 350-460 CA, kh 7-10, mg around 1350. It doesn't have to be perfect to get some PE, but pretty close to it.
Tell us about your tank params etc. That way we can help you out. You may also may want to see if they are extended at night. If they are growing, have good color and extending at night I wouldn't worry about it. Nightime is when they feed anyways. Although daytime PE is cool looking it's not manditory to have good sps health.