Since the iPhone X was the first one featuring Face ID, there are more posts and guides about malfunctioning Face ID there than anywhere else, so it might be worthwhile to browse through them and see if anything looks applicable to your situation. Here are the ones I found.
* [guide|128932|How to Fix iPhone X Face ID Not Working After Screen Replacement - iFixit Repair Guide|new_window=true]
* [guide|128928|Troubleshooting iPhone X Face ID Not Working 'Move iPhone a Little Lower/Higher' - iFixit Repair Guide|new_window=true]
* [guide|128388|Why Face ID Not Working After Repair - iFixit Repair Guide|new_window=true]
* [guide|102195|Why iPhone X Face ID not work after repairing - iFixit Repair Guide|new_window=true]
* [[iPhone X Troubleshooting|iPhone X Troubleshooting - iFixit|new_window=true]]
* [guide|144110|iPhone X Face ID Not Working Fixed By Replacing Earpiece Flex Cable - iFixit Repair Guide|new_window=true]