You can work with them for half a year, i had to do this with one ray, but the 8 others took it after a week or two.. Cut nightcrawlers up in small pieces, put them in box, cut some shrimp in really small pieces, put them in the same box, add a little water(often there is water in the shrimps so you dont need to add), put the top on, shake it a little, let it stay in your refrigerator overnight. Then, ONLY feed the shrimp, not the worm pieces.. Do this for 3-4 days, if they dont take it, bulk them up again on nightcrawlers for 2-3 days, and start it all over again!!
IMO its the only way!