Most likely due to one of the below two reasons:
1. There are no duplicates within the folder you scanned. You may think that there are duplicates, but remember the file has to have a corresponding identical duplicate with exact hash values matching (for example, you might have two images that look identical, but their meta data could be different and therefore they won't be considered exact duplicates).
2. There are duplicates, but the app is ignoring them as they are below the minimum size threshold set in the app's preferences (default = 50kb).