There are some reasons why the contact is not found from MailChimp after Pipechimp has run the sync.


1) MailChimp rejects the contact adding

https://mandrill.zendesk.com/hc/en-us/articles/205582957-About-Rejected-Emails


2) There are duplicates in Pipedrive

MailChimp list can contain only unique email addresses. It means that if multiple Pipedrive contacts have the same email address, only one contact is created to MailChimp.


3) There is no email address set to contact in Pipedrive

If email address is not found, a contact cannot be created to MailChimp.


4) The email address is invalid

Pipedrive doesn't check is the email address correctly formatted. If it contains for example double dots in a row or some other special characters, it is rejected by MailChimp.


5) The owner of the contact in Pipedrive is different than the user which is using Pipechimp

Pipechimp syncs only the contacts owned by the user which uses Pipechimp. This is changing in the Pipechimp 2.0: https://www.pipechimp.com/pipechimp/we-are-building-a-new-pipechimp-2-0/


6) Someone has unsubscribed from the mailing list

If someone unsubscribes from the mailing list, it will decrease the contacts count in MailChimp list.


7) Cleared contacts decreases the contacts count

https://kb.mailchimp.com/lists/manage-contacts/about-cleaned-contacts