Well is it a piece that you just got, if so did you acclimate them properly, looking at the amount of posts you have I am assuming that you have been doing this for a while. But as you probably know improper acclimation will do that. Then there is always the placement to much water flow, too little water flow, maybe you did a light change recently. Maybe you moved them closer. This is all stuff they are going to need to know in order to help you out with the situation. I know that i have had a whole colony do that and they came right back. Post some pics. Maybe that will help us out.