My hermit crabs keep coming up dead! What should I do?

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In the past two months, three out of the four crabs that I bought just before Christmas have kicked the bucket. One died after he molted, one died while after a bath, and one died trying to get into water dish. The humidity and temperatures have been staying at the normal, but something has made them die anyway. I bought calcium fortified food and it turned a little moldy so I threw it away. They have other shells to climb into, but they never tried. The sand is a least 3 inches deep and they buried themselves regularly. What do you think may have happened? Are yours active? How have you built it's cage? Do you have more than one?