CRM to Campaigns Sync not importing all my CRM Contacts

CRM to Campaigns Sync not importing all my CRM Contacts

I've observed that all the contacts from my CRM are not syncing into my mailing list even after following all the guidelines for automating the sync process.

Do I have to do this manually? Is there a field in the CRM contacts that the sync looks for before merging new contacts in the existing list?

Thanks