If you have ran a scan and no duplicates are found, don't worry. You either genuinely don't have any duplicates, or you need to tweak the criteria a little.
Common settings which can cause no duplicates to be found are:
Subfolders not being scanned.
Are you scanning the subfolders of the input folders you have listed in the 'Folders to search' list? Be sure the 'Scan subfolders' column is set to 'Yes' (Or the 'Scan subfolders' checkbox is ticked if using version 3).
System folder being ignored.
Are you trying to scan a folder/subfolder which has a Windows 'System' attribute? Uncheck the 'Skip system folders' setting. Always be careful not to delete system files unless you are sure you don't need them.
File filter restrictions.
Check your file filters on the 'Search criteria tab'. Generally you want to scan everything ('All files') and exclude nothing.
File size/date restrictions.
Check you don't have any file size or date restrictions in the Criteria tab. Try 'Any size' and 'Any date' if not sure.
Restrictive criteria.
Make sure your search criteria isn't too restrictive. If searching by content, you don't generally need 'Same file name' or 'Same created / modified date'. If you have 'Same created date' checkmarked this won't generally find copied duplicates as they usually have different created dates.
Other folder settings.
In the 'Folders to search' list make sure that 'Find duplicates' is set to 'Yes' if you want to find duplicates within that folder. If 'Master' is set then bear in mind that only files that duplicate any folder with the 'Master' flag will be shown.
Duplicate pictures not found.
Be sure you are running in 'Image mode' to find similar, resized or rotated images. Otherwise only exact duplicates (same image, tags, size, etc) will be found.
If you want to find RAW camera image formats such as .cr2, .dng or .raw you need to checkmark 'Raw image support' in 'Advanced search options' on the Criteria tab.
Office Documents or PDF files not matched
Sometimes documents such as Microsoft Word files, while seeming identical, may not match in 'Same Content' mode. This is because the files may have been loaded/saved at different times and so have tiny date changes embedded in the file. This prevents an exact match. One way round this problem is use a 'Similar Content' scan with a very high setting (98 or 99%). Always be careful to double check the files before removing!
