shrimp is good, I feed it to all my rays but it takes a while sometimes to get wild caught rays trained on shrimp.
They usually will except blackworms, you can try blood worms (my new born pups will eat 'em)---rays will usually except cut earthworms as well then whole worms as they get larger.
Retics are known to usually be picky eaters at first. I've never had retics but I hear/read they readily eat blackworms. Find something it'll except, fatten it up then start trying other foods.
Also remove any uneaten food.