I've personally always had success feeding my shrimps and crays JBL Prawn Novo, although with that being said, my crays will take just about any food including bits of mussel, clams and market prawns...
How often do you feed them? Maybe trying skipping a day or two and try again. Also, what is the temp of your water? Lower water temps can some times mean slower digestion.
they will anything from dead fish, plant matter, any fish food, snails, earthworms, live fish if they can catch them. Try something like a snail or earthworm. That should really tempt them. Once they start back eating they should have there normal appetites!
How often do you feed them? Maybe trying skipping a day or two and try again. Also, what is the temp of your water? Lower water temps can some times mean slower digestion.