Outside of the initial configuration, records are not evaluated for synchronization to the Marketing Cloud until the Modified Date is changed via a record update.
NOTE: Updates to formula fields do not trigger a change to the Modified Date on a record, and will not trigger a re-synchronization.
Performing an manual refresh will trigger a re-sync of all records in the CRM, regardless of the last modified date. If you find you are missing expected contacts within a synchronized data extension - such as contacts that have been deleted in the Marketing Cloud but not Salesforce - follow the steps below:
Implement Synchronized Data Sources Best Practices
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