Molting is extremely stressful for a crustacean and anything that weakens an animal can cause it to die while trying to shed its exoskeleton. This can be due to poor water quality, disease, parasites, injury, or just plain old age. I would guess that at least 90% of the animals I lose die attempting to molt. When this happens, it may be indicative of a problem, but not necessarily. If you are confident that your water parameters are good and there were no signs of shell disease or injury, you will probably not find the "cause".
I occasionally see reference to iodine supplements being used to prevent molting problems. I know of no data on this for stomatopods. In decapod crustaceans, a lack of iodoine can cause problems, but with a typical varied diet of shrimp, krill, snails, etc., this should not be a problem. I generally worry more about my animals getting enough calcium which is why I like to include freeze-dried krill or shrimp (with the shell). I also use a supplement such as Selco. It seems to help, but I honestly can't say why.
One more thing. Watch out for the build-up of heavy metals. These can be lethal and you usually see the effect when the animal attempts to molt. Most aquarists now use some sort of de-ioniized water in mixing their salt water or for salinity adjustments. If you don't, heavy metal concentrations over time can build up in a tank to lethal concentrations. And water is not the only possible source of this contamination. Years ago, we used to feed our stomatopods and octopus snails and crabs from San Francisco Bay.. My success at keeping animals in the lab for more than several months was not good. Analysis suggested that we had a heavy metal problem, but we were using de-ionized water. It turned out that the heavy metals (as well as other pollutants) were coming in thorugh the food chain. We now use prey from outside the Bay with much better success.
Roy