duplicates found but not displayed

When launching duplicates search, I have a DigiKam message mentionning 
severai hundred duplicates in my database (or less if I select a 
folder), but none is shown in duplicate window nor in the thumbnails view.

I know there are many, as I imported from different sources.

Any clue on how to fix it ? Are there some parameters I did not 
configure properly ??

Thanks
 From a beginner with DigiKam