After the period of each sticker album ends, add an additional period of a few days for the users that didn't finish their albums to give them an extra chance of completing them and during that extra few days period, the users that were able to complete their albums in time receive a perk, like a chop chop event, a bonus of energy, gems, or crates, or any other perk to make things balanced. After the grace period ends, everyone starts a new album and cycle repeats. This way, it will still be attractive to complete the albums on time. Or you can even reduce the reward for completing the album during the grace period as a form of penalty for not completing it on time.