This just started a couple of weeks ago. I'm running Outlook 14.4.1 on Mavericks 10.9.3, but before I updated, the problem occurred. The sub-folders just disappear after Outlook is open for a while. There appears to be no specific time and my RAM situation doesn't seem to be an issue. The folders come back if I quit and relaunch Outlook, but nothing I do makes them want to stick around. 

Rebuilding the database doesn't help; neither does dumping .plists or prefs. Any ideas?