Each time the reservation expires, the reservation period for that user is shortened. For example, the campaign settings allow a 5-day reservation, but if a user has missed 50% of reservations, the reward can only be reserved for 2.5 days.

Users can cancel reservations at any time, and there is no penalty for doing so. Just go to REWARDS / Reserved and click on '...' for more options. There you can select "Release" to cancel that reservation.