While there are many reasons an email may not send at the time it is scheduled for, it should still show in "queued" status while it is waiting to send. If the email shows a status of "audience generated" after the schedule time has passed, this could simply be a time zone issue. Times in Luminate Online are generally Central Standard Time. Next to the delivery click on "Review" and the scheduled time for the delivery will show the time zone. Make sure to adjust the timing based on the time zone if necessary. If this does not explain your issue, please contact support to investigate further.