I agree with rvrrays method,
for me wad i do is, pump the ray up for say a week, feeding it at least once or twice a day with its favourite food.
then aft that week, starve it for 2days, then drop the new food in. In its hunger it might accidentally swallow the new food, either that or it will eat it well.
if it fails, feed it its favourite food for 2 days, then starve for 1 day and retry again..
good luck! usually healthy rays are easy to get them to eat new food!