Reindeers tend to get stuck carrying trees while the storage is full, resulting in no sleighs being able to depart and no more storage space ever getting freed up. Bit of a dead lock that has to be manually solved by placing more storage or reindeer schools (and sometimes a bit of luck if all newly availablereindeer decide to still ignore the sleigh).
That being said, excellent job. You've successfully wasted 8 areas worth of my time.