Running the provisioning task and viewing the report will show "skipped system mailbox" therefore the mailbox will not be archived.
Skipped system mailbox
This will occur when the mailbox is configured as a system mailbox on an archiving task.
On each Enterprise Vault Server, perform the following steps:
1. Expand Enterprise Vault Servers | Tasks
2. Go to the properties of each Mailbox, Public Folder, and Journal task
3. Choose the Settings tab
4. Confirm that the System Mailbox field does not contain the mailbox in question
If it does, it will need to be removed and provisioning run again to correct the issue.