Hammer coral (euphyllia ancora) is used to be natural water parameters monitor. If something's wrong in water chemistry corals from genus Euphyllia may not open. What do ya mean by saying perfect? What are your major water parameters: PH, ALK, Ca, SG, PO4, SiO2??? Try to remember if anything was done prior to this. Visually check the coral: any mucus, brown stuff?
These are just suggestions that may help you locate the problem.