Verify that your background services and outbound mail services are running. It's also worth checking the event log to see if there is an error that has been triggered. Also, when you open the "reminder" from the ticket, so you have the "Send Message" action? If so, it may be worth manually selecting that to make sure it in fact does send and remove the Active check box.
Is the "User" attibute a default one or one that was user created?