Well honestly that was my first thought. Iodine is an important component for them to molt properly. If they do not have iodine, they cannot shed their old smaller carapace to accommodate their growing body: think of growing in a nonexpendable suit of armor. Their internal organs are crushed; therefore, they die. I would go with a premade reef iodine solution as directed unless you are very familiar and precise with Lugols. You could probably get by with half the recommended dosage every two weeks IF you're not running a large load of carbon. A little more info about your system would be helpful to trouble shoot.