Face detection not working for one file

[Darius1968]

So, I had a group of 30 files for which I ran (via the right-click menu) Face Annotations and People>Detect faces and create Face Annotations, and of those 30, 29 were annotated, allowing me to double-click the annotation in the viewer to associate it with a person.  However, the one that didn't get detected continues to not respond to the above ...Detect faces... command.  Why is that and what can I do about it? 

[Mario]

Maybe the AI is not able to identify a face?
Very small image?
Very small face in the image and you did not change the detection settings to optimize for small faces?
Only one eye visible? Face obscured?
Many reasons as explained in No Faces Found?
Open the file in the Viewer and zoom in a bit. Press <F6>.
If no face is detected, add a manual face annotation.