Sometimes Personal Discount codes are not appearing in the emails sent by Mailchimp.

To prevent your emails being sent without a discount code, make sure to add the “Discount Code is not blank” conditionto the automated email in Mailchimp. This way, you can be sure that not-including-code email will never be sent to any customer. Mailchimp will send the email only after a discount code has been generated and assigned to a contact's profile.