Air's facial recognition technology is highly accurate, but face detection depends on the image and whether Air can detect a face in a supported image context. The People panel appears when a face is detected, so some images with people may not show a detectable face for tagging. You can always associate a detected face with a person.
If you expected face detection on an image and it is not appearing, contact Air support using the messenger chat on this article page or email help@air.inc so we can check for a known Facial Recognition issue, missing enrichment, or plan availability.