how long not more that 25 to 30 cm it will not work plus your face sensor will capture the background of you are too far away from sensor the box in sensor on screen should be filled with your face the box you seebshould outline your face hence going to far will compromise your face id if any other problem try resetting device and again re register your face id. try fingerprint or password or pattern or pin face id is most insecure compared to these options 👍👍👍👍👍👍👍👍👍
