In Outlook 2011 for Mac, the status bar displays the following message:
Inbox: Downloading Attachments
The status shows 1/20, and after 20/20 is reached, the status returns to 1/20, and the process is repeated. You may notice this behavior if you recently configured a new Outlook Identity, a new account, or emptied the cache of a folder, and Outlook is synchronizing the data from the server.
This behavior is by design. When Outlook 2011 for Mac downloads data from the server, Outlook first downloads the email headers and message bodies, and then Outlook downloads the attachments for each folder in batches of 20. During this time, the status displays Inbox: Downloading Attachments x/20. Then, the number repeats from 1 to 20 until all the attachments are downloaded.
Allow for enough time for Outlook to finish synchronizing and downloading all attachments. If you have many items that contain attachments in the folder, this process takes longer.