Ring Doorbell 3 plus not detecting people

I have set up my doorbell. It detects people leaving the house and when I drive in the driveway with my car. However, it doesn’t detect people walking in to my drive. I have people detection on, sensitivity up to maximum. The postman wasn’t detected at all. What can I do to sort this? If I can’t get this to work, i’ll be sending the doorbell back for a refund. Very disappointing.