Wow, I never had luck with leaving unmounted rics in the sand. They would definitely go missing somewhere in my tank. It surely would not hurt anything to put a rock up next to or under at least half of it so it will have something to attach to besides just sand. Then once it attaches, you can move it, if desired. It will be attached to what sand it is touching now, so it might take it a while to decide to release the sand stuck to it and attach itself to the rock (previous experience). Just be patient. Another option would be to find a nice indentation in a rock that's about the same size as the ric, not in a high flow zone, and let it attach there.