If you find that Customer Data is not created or updated as expected in Mailchimp please read below for the most common reasons this may happen:
Mailchimp Minimum Required Data
For a customer to be created in Mailchimp at minimum a customer email address is required. Some addresses are rejected by Mailchimp as invalid, such as email@example.com. Please be certain that the customer data you're expecting to sync has a real and valid email address.
Required Custom Fields
If a custom field in Mailchimp is marked as required it will prevent the sync of new customer data to Mailchimp. Please make sure no custom fields are marked as required in the list used for the integration.
Custom Fields with Duplicate Names
If a custom field has been added that has the same name as a default field, please re-name it. Custom fields in Mailchimp that share the same name as a default field (such as "Address") will prevent the sync of customers to Mailchimp.