I'm not sure why they sometimes close up and refuse to open for days. I had black suns that were doing ok until I place a torch coral nearby. I thought the distance was far enough but the black suns seem to disagree. After a few days, the side nearest the torch refused to open even when feeding. In the end I had to move the suns to another location. And they gradually start to open again. So check if you have placed them too near to other aggressive corals like torch, bubble, hammers, etc which can extend long tentacles at night.