After importing a .pst file from an IMAP account, some folders may appear empty in Outlook though they contain messages in OWA. You will need to change the view for these folders in order for Outlook to show the IMAP messages.
1. In Outlook, select the folder and then click the VIEW tab.
2. Click the Change View button and then select the IMAP Messages button.
To set the view for all folders, refer to the following article: