So, reading through this thread and I agree that having 0 nitrates is a problem. What other coral do you have? Do you have a lot of algae in your tank? It's *possible* that your nutrients are sufficient but the coral and algae are utilizing them all. How often do you test? Did nitrates use to be higher? Could the recent test be wrong?
As for the hammer itself, sometimes when a colony gets too big, some of the heads suffer, due to not getting enough light, etc. It's possible that the head in the middle of the colony died simply because it was too crowded.
I hope you find some answers and that the coral does well :)
One other thing: while dipping is often a good idea, remember that any time you move a coral, it stresses it. With fleshy coral especially, if you don't allow them to deflate before removing them from the water, the weight of the polyps can actually cause damage, which can then turn into an infection... Just something to keep in mind.