AMP searches only on selected fields, among them: username, MAC address, IP address, device name, Group, Folder, Location, and Notes.
If the search string does not appear in one of the searchable fields in a record you are looking for, that record will not appear in the search results.
Also, in the case of user records, records may appear in your search results that don't obviously match your search string (that is, the search string doesn't appear on the results page). This is because the string appears somewhere in the history for that user. If you click on the link for the MAC address in one of the records in the search results you can see the history for that MAC address and you should see the search string appear on that page.
The results page displays sections for matches from APs/Devices, Users, Folders, Groups, Rogues, and Tags.