Keep them clean. Buildup of unwanted detritus and waste between polyps can lead to bacterial and fungal infections. Proper random water movement is essential, and frequent "basting" can go along way to insuring the health of a colony.
I have a painter soft brush, I use to clean the colonies. When I see a few not opening up or no babies are being produced, I clean the colony with a soft bristle brush, in a few days it starts opening again. This will often eliminate the zoa from melting