Are you using a screen protector that covers the FaceID sensor array? If you have, remove it.
Screen protectors that do not have a cutout for the FaceID sensor array may interfere with (or completely inhibit) operation of FaceID.
If you do not have a screen protector fitted, carefully clean the screen bezel that incorporates the forward facing camera. The FaceID sensor has separate IR Camera, IR Dot Projector and IR Flood Illuminator - in addition to the visible-light camera. You can clean the camera array using a clean lightly dampened microfibre cloth; cleaning materials used to clean spectacles are ideal.
If FaceID is sensitive to lighting or angle, this would strongly suggest that the sensor array is either (a) obstructed by an incompatible Screen Protector, (b) the sensor array is not clean, or (c) the sensor itself has fault. If you are unable to resolve the problem, you would be best advised to visit the the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad:
Genius Bar Reservation and Apple Support Options - Apple