How much light are they getting? I've found these to be pretty hardy but they'll shrivel up if they're getting too much light or flow. When this happens in my tank, I typically move them to a shaded area and see if they'll recover. Typically they do and then you'll notice them "reaching" for more light and can move them out some.
Note: If you have LED lights, don't assume that the sandbed is the weakest as far as light intensity goes. I have a specific 4x4" square on my sandbed that gets more striaght PAR than the rocks at the top of my tank. Seriously. I think it has something to do with the "cone of light" that each fixture puts off crossing at a certain point but I'm not positive. I just know that I was surprised with some of the PAR meter results I got.