Having too many custom merge fields in an email can cause the email not to send. Limit each email to no more than 15 merge fields to prevent the emails from not sending. The test email is able to be sent because the merge field information is not pulled during a test email.

The email could also be delayed in the closed state if the Receiver Query has too many criteria. Limit the criteria of the Receiver Query and avoid using successive Receiver Queries where the Receiver Query pulls in past email's Receiver Queries and that email pulls in other past Receiver Queries, etc. creating numerous criteria. This can either keep the email from sending or delay the sending of the email.