It is possible they are not completing the entire activity.
The Popcorn Library involves reading the story twice. If a player logs out before completing part 1, they have to start at the beginning of the book the next time they log in.
If a player logs out during part 2, then they have to start at the beginning of part 2 next time they log in.
In the event your completing both readings of the book and are still repeating the same book, please contact email@example.com for assistance.