I delegated my coins to one of the masternodes. A day passed and I got no reward. Why?

Check through one of the services

http://monsternode.net/validators.php
https://minter.stakeholder.space/validator/globals/

whether the masternode is active and whether it is included in the list of validators. A masternode could be temporarily disabled for configuration or excluded from the list of validators by the size of the stake because of the unlimited volumes of test coins.