Niffler won’t respawn

I’m currently doing the A Thief in the Night quest and the nifflers won’t respawn.

I’ve tried waiting in and out of range of the purple zone, waiting for different times of the day, went off collecting potion ingredients, fought nearby spiders and goblins, went back to Hogwarts and Hogsmeade, did 3 other side quests and a main quest, flew around on the hippogriff, overall closed the game and came back to it the next day.. nothing has worked.

HELP? Am I not noticing something? Is there a weird trigger I’m not aware of? I’ll feel ridiculous if it was something simple.