The most common cause of this problem is If you aren’t seeing a shared mailbox in your Outlook client after 24-48 hours of its creation, you may have a corrupted OST file. Although repairing that corruption can be done via the KB article Rebuild the Outlook Cache to Fix Corrupted Data Files, the easier and more long-lasting solution is Use these steps to configure Outlook to not cache shared mailboxes.:
In the Outlook client, click the File tab in the upper-left.
Select Account Settings, then select Account Settings again inside that menu.
In the Account Settings dialog, click Change from the menu on the Email tab.
Click the More Settings box.
Select the Advanced tab.
Unselect the checkbox for Download shared folders.
Click Apply and restart Outlook.