Categories disappearing from Outlook inbox at random

I use an intricate category system for keeping my inbox organized, but every so often, I will open Outlook and all my category assignments will be gone from the messages I categorized. The categories themselves still exist in my "Edit Categories" window, but they no longer are attached to any of my e-mails. This has happened numerous times since I began using Outlook from Office 2011, and there is no rhyme or reason to what happens, and I can't seem to get them back so I always have to go through and redo all the categorization.

If someone would please advise, I'd appreciate it. Thanks!