Hi Brett,
From your description, the issue could be caused by that you haven’t applied an archive policy to the mailbox after you turned on the “In-Place Archiving” feature. To do that, please try the steps below:
- Login in OAC (Office 365 Admin Center) as an administrator, click “Admin”-“Exchange”-“recipients”.
- Double click the mailbox, navigate to the “mailbox features” tab, in the “Retention policy” tab, select a policy and save.
Then it will create an archive mailbox in the user’s mailbox and will move the mail items which meet the policy to the archive mailbox. If you still can’t see the items which will be expired, I suggest you create a new Retention tag with a short period in it, then include it in the policy and assign the policy to the mailbox.
For more details, please refer to the links below: