Occasionally, plugin users report that their emails are not being delivered to their subscribers/users. This is can have two causes:
1) Your website does not receive daily traffic. The WordPress system is using a function called ‘WP cron’ to send emails. Whenever someone visits your website, the ‘WP cron’ function is triggered and will send emails that are in the sending queue. If nobody visits your website for a few days, then the ‘WP cron’ function won’t be triggered. Hence, your emails are not sent as expected
Solution: set up a cronjob with your hosting provider. The cronjob should just request your homepage every 15 minutes. This way, the ‘WP cron’ function is tiggered regularly and will send your emails on a frequent basis.
2) Your server runs into timeout issues while sending your email batches. Some webhosting environments limit their script execution time to 30 seconds or less. If you have many subscribers/users on your list, the sending process will take longer than these 30 seconds. The script will then be interrupted and stop sending emails.
Solution: please refer to this short explainer on how to properly set up the email queue.