Outlook subfolders mysteriously go missing…

I currently categorize many of my e-mails into subfolders inside folders on Microsoft Outlook 2011 for Mac. Trouble is, in past couple weeks, the subfolders seem to have disappeared. No little arrows to expand folders either.

The emails are still there, because I can locate them using Finder.

But I can't actually access the subfolders themselves.

And here's the strangest thing: I can view the subfolders when I first open Outlook until I actually run send/receive mail, at which point they all disappear! If I close and then re-open Outlook, they appear again.

Does anyone know what might cause this, or how to fix?

Thanks!