hammer coral

[ILIKECOUGARS]

I have hammer coral that will not fully expand, all other coral are doing fine, (frogspawns, candycanes, brain coral, torch coral). I pick up this coral about 2 years ago with 2 to 3 heads on it and now has 18.
Change the carbon in the reactor, did a water change with no results. It's been like this for about 4 weeks now. As I mention all other coral are doing fine.

Salinity 1.026
Cal 420
Alk 2.7
Ph 8.0
No3 25

Any idea what could be wrong with it.