I have a sarcophyton that has been doing the same thing. I tested my water and everyting seemed to be in line with pH, CA, Alk, Temp, SG, Nitrate, Phosphate - all in check.
Hmm?
For almost two weeks the sarcophyton (and some zoanthids) did not open. After I tested the water I decided to change out the carbon with a Poly-filter pad. Low and behold the pad turned greenish brown after a few days indicating that it was removing ammonia...which I did not test for. :sad2:
So then I tested for ammonia and found that there was a very low level of ammonia in the tank (after running the Poly-filter). I can not explain it. The system must be experiencing some die-off from live rock or maybe a snail or two?
So now the sarcopyton and other corals are begining to show some polyp extension since the ammonia level is dropping. I would think about checking ammonia, nitrite, nitrate and phosphate in your system to see if they are at elevated levels.