polyp extension

[24storm]

What causes polyp extension in SPS? I have pieces that are right beside each other in the same flow. One will have great extension and the other not so good.
It is not always the same corals sometimes some will retract the polyps for days then come back. Do i need more flow? I have 2 Tunze 6100 and a Tunze wavebox in a 220 6 foot tank.

Any advice would be appreciated.

Thanks Keith