1. Check your Face ID settings:Before using Face ID, you should make sure that it is already set up and the feature you are to use is turned on. To check the settings, you can go to Settings > Face ID & Passcode.
2. Make sure nothing is covering the camera:
Do not wear phone case or screen protector on your iPhone X! Plus, check if there is dirt covering your iPhone camera.
3. Make sure that nothing is covering your face:
Face ID does not work while wearing Face mask!
Face ID doesn’t work with some of the sunglasses.
Do not wear make up.
4. Do not close your eyes:
If you close your eyes while using Face ID, then your facial recognition will be failed.
5. Do not eat food while using Face ID:Many iPhone X users pointed out that Face ID is useless to them when they have food in their mouths.
Solution 1: Make sure that you're facing the TrueDepth cameraKeep your face close (10-20 inches) to iPhone X camera and do not stand in backlight so that your face can be recognized.
Solution 3: Rest all settingsIf your problem still exists even if you have tried the solutions mentioned above, you can tap Settings > General > Reset > Reset All Settings to reset your iPhone X to solve the problem. This method will definitely fix the iPhone X Face ID issue unless your iPhone has other technical problems.