I have a lunar wrasse and have had it for about 8-9 months. I have a very fine white sand and every night after lights out he buries himself. However, I'm am unaware of your question. Are you asking why yours DOES bury himself or are you asking why it doesn't at all? If its not, then it could be the crushed coral substrate you have. If it is too heavy or coarse, it may not be "comfortable" for it and after all that is the reason they bury themselves. For comfort and protection as well.