If it won't open up, it's not liking the placement - try different flow and lights for a day each spot until you find somewhere it likes. In the wild they are usually in the sand in mild/medium flow, but depending on your lights it ay need to go higher - I've seen a pvc pipe capped and filled with sand placed up high on rocks work too. Try to feed it a little something, maybe brine shrimp or mysis. I use a small pipette and gently squeeze out a few bits onto each head, closes over them and hoes it down.