BigPete why didnt you come to me and ask me this question?haha you know I have hammer corals!
Make sure you got low/moderate flow. In my tank, they have good flow so that there branches wave in the water but not direct flow. If its high flow, their branches get small, if its low flow their branches gets long.
I think what your having is a water quality problem. I always noticed with mine when the water quality gets low they start to shrink. OR
Have you added any chemicals directly into your tank or try to adjust the ph in some way? That also makes them not open up for a while.
Here is the specs that I keep my hammer corals..
ph 8.2
salinity 1.023-1.024
low/moderate flow
lighting: gets direct light of my 36watt PC light and it also gets metal halide lighting a little bit.
placement:Center, middle of the tank next to my frogspawn
feeding: I feed it phytplankton cube every week.
Keep your water quality good, ph and salinity balanced! Hope this helps. The seem pretty easy to keep, DId you make sure that when you bought it, it wasnt damaged..or that none of your fish or crabs have been picking on it?