Just like with any new food black worms may take a few times before they get comfortable enough to try it. So make sure you arent putting in too many at a time until they get comfortable enough.
The mandarin's main source of food is pods. You may have a hard time getting it to eat anything else. Thats why its recommended to have a mature tank with a thriving refug to produce the pods the mandarin needs. They eat a crazy ammount of pods a day.