Flashcards status not saved

Over the past month, whenever I revise my flashcards, I keep getting a message saying '[X] changes didn't sync' (where X is the number of flashcards I studied in that session). At first I ignored it, but I've noticed that the next time I go to revise, the flashcards I already reviewed show up again instead of being marked as done. As a result, I keep getting stuck re-revising the same flashcards instead of progressing to the ones further back in the set.