They are best on rock. They are "boring clams" and do better there. In the sand is okay if you have good lighting, but I'd still put something (small LR, empty
clam shell) under them to attach to. This will help them protect the byssal port from predators (ie. your pyramidellid snails). Also, because Crocea have more delicate gill structure, they really don't like ingesting sand. Yet another reason to have them on rock.
If they are moving, and not attaching, check lighting and flow. If they are being bumped by fish, snails etc. build a little rubble pile around them until they attach.
If you have little white snails, they're probably pyramidellid snails and you should do something about that. Manually remove them from the byssal port and toss them in to the snow

check it DAILY and remove them. A decent wrasse will eat them too.
Dave