How long have you had it? It may just be settling in if it's fairly recent. You will want it on high crunch foods for beak control, but try Canadian night crawlers. Unless it's belly is sunken in drastically I would just keep trying shrimp.
Shell on shrimp, frozen crayfish, clams on the half shell, crab, snails, and Canadian night crawlers was basically my mix. I soaked them in vitachem except for the worms. I bred marbled crayfish in a tub for a little bit, but mine out ate the breeding abilities.
Keep an eye out for signs of internal parasites to.