Probably one small appropriate selection at the Options section is missed out !
After you have ticked the relevant “Match by” parameter checkboxes:
Click on the <Add> button, a pop-up window "Include" is displayed.
At the Include section, click on the <Browse> button to select a drive or folder which you wish to search for duplicated files.
At the File Types section, leave the radio button defaulted to "All Files" or select "File Types" to key in the desired file type(s) to search.
At the Options section, select "Include files and subfolders" from the dropdown list.
Click on the <OK> button to exit the pop-up window.
Click on the <Search> button to begin the search for duplicate files.
