If a face is not easily discernible in an image, Lightroom may not be able to detect it. You can draw face regions manually in such cases.
- Click the Draw Face Region icon ().
- Draw a rectangular face region on the image.
- Enter a name to tag the face.
- If necessary, tag more faces.
For more information please refer to : https://helpx.adobe.com/lightroom/help/face-recognition.html
Please let us know if it helps.
No, that doesn't seem to work if there are already identified faces. At least I cannot get a rectangle to draw as clicking the "face draw" icon only "lights up" the faces that are already labeled.