Newsletters are not being sent
This is most likely due to cron not running on your site.
Cron
Please use a tool like WP Crontrol to check if cron is running.
Unless you have an alternative to WordPress Cron (as is the case with WP Engine), you should not have the DISABLE_WP_CRON constant set to true in your wp-config.php.
WP Engine
If your site is running on WP Engine, you must ask their support team to enable Cron for you.
See WP-Cron and WordPress Scheduling – The WP Engine alternate cron is a true cron that runs every minute on the minute, checking for and activating scheduled events. This is not enabled on your server by default, so you will have to request to have it enabled via Support. Once it has been enabled, it will ensure that the events are run as scheduled.