Front Step Theft Not Recorded by Video Doorbell

A flag was stolen from the front of my house overnight.  To grab the flag, the thief would have had to stand directly in front of my SimpliSafe video doorbell.  Yet the doorbell did not record anything.  I do have a recording of the mail being delivered at 6:14 p.m. (and the flag is visible in that video).  But nothing of the theft.  I had the camera set on "medium" sensitivity for night vision, "people only," with no activity zone excluded.  Any thoughts on why this activity would not have been recorded?