This is expected behavior. For performance reasons, the search bar only supports a limited number of fields, which are listed on page 85 of our online documentation: http://www.lansweeper.com/documentation.pdf
Note that you can create a custom report that lists the FQDN and filter the report within the report results to perform a "search". To create a basic report, open the report builder under Reports/Create New Report, tick the FQDN field in the tblAssets table, submit a report title and hit the Save & Run button.