Why is no reminder send via Notification Manager?

When a guest did not receive a payment reminder, the user can perform the following checks: 

• Is a reminder entry added in the booking based on the reminder process?
• Is the reminder process executed before the scheduled tasks of Notification Manager? 
  • If yes: Raise support request to Maxxton to investigate
  • If no: Time of creating reminders is after scheduled Notification Manager task, because of which there are no reminders sent per scheduled NM task.