well lets see if this helps.
Ghost shrimp are ok, but not a everyday food. they wont get all the vitamins they need just on ghost shrimp. I have 4 Motoro rays. They love ghost shrimp but thats not all I feed them. You may not want to cut up night crawlers but its good for them. A ray is a fish, fish love worms. and it has some of the vitamins they need. Mine dont seem to like bloodworms either, and hate frozen foods. but remember, frozen foods also dont have all the stuff they need either. when you freed seafood, it loses some minerals fish need.
So try this. Ghost shrimp ok but not just that. Night crawlers are good and easy to find and cheap. Live blackworms (some good aquarium stores have them) are good also, cheap and small enough for ray to eat easy. Just get a turkey baster and feed the blackworms to the rays with it. Some frozen food ok, but not just that only. If they wont take uncooked frozen shrimp or frozen krill, try tilipia fish. cut into small pieces and feed. but they prob wont like it to much either.
remember, rays are preditor fish. they like live food more than anything.
so, switch it up. ghost shrimp, blackworms, tilipia time to time, night crawlers.
start there and see how that works.