Re-scanning folders to detect faces previously marked as ‘don’t show again”

scanning and tagging of my photos is completed. i choose don't show again on many faces. Now I want to re-scan the photos to detect those faces so I can tag some of them. I changed the option to scan faces in file/options. Closed the app, reopened file/options and changed back to the scan option, closed and restarted. that didn't work. any other ideas on how to re-scan my photo folder and pick up faces that were previously detected but not been tagged with a name?