There could be Problem with Software, Hardware or External Factors etc.
1. Software Update:
One of reasons could be software. Please make sure you have latest version installed.
Camera Calibration
One of reason could be problem with Camera calibration. You need to do Camera Calibration.
2. External Factors
Weather Conditions:
If there is fog, snow, heavy rain, low visibility, lane line are not visible.
Road Conditions
One of reason could be that you are driving on narrow roads.
Cameras or Sensors are Blocked and Blind because of Dirt or Debris
One of reasons could be your car cameras are blocked because of dirt or debris.
You need to Clean cameras.
Not driving in Speed Range
One of reasons could be that you are not driving in speed range.
Bright Light
One of reasons could be bright light coming from oncoming vehicle headlight or sunlight, which interfere the view of camera.
Hardware
Sensors (if equipped) are affected because of other electrical devices that produce ultrasonic waves
Tesla "Lane Departure Avoidance feature unavailable" Guide to fix
This guide might be interesting for you. Please read more details in another guide. Link
Source: Tesla
