This is reported as a Microsoft Bug as seen in the link below and mainly effects people with exchange.
"This problem is caused by a bug in Microsoft Outlook. The information for the userdefined fields is stored locally on the user's PC and the search is done using information on the server. If you switch to offline mode, it will work. If you do the search while online, the userdefined fields will not be displayed. There is no fix for this issue until Microsoft issues a bug fix. Switching to offline mode is a work-around that can be used in the meantime."