Sand Sifting Stars are murder on sand beds. I'm not happy to read that it is dieing but now that you have come here with the problem, please don't buy another one for a long time. If after two years you feel you must have one, keep it for a few months then give it to another large tank for a few months, and so on.
For the record, Jawfish are also murder on sand beds. I strongly suspect it also died of starvation.
Personally, I prefer cucumbers. They don't seem to eat as much of the life in the sand, they move slower eating less...
My 2 cents.