When you send out an email message, there is a built-in filter that filters out anyone who has ever received that exact message before. This is generally very useful so that you don't accidentally send duplicate messages, but can cause confusion when you intend to resend the same message to the same audience. To send out your corrected message, make a copy of the message and setup the delivery process again. Once you copy the message it will assign a new message ID, and it will not filter out past recipients.