1. Check the video cable connections to make sure the connections are firm and secure.
2. Check if the video cable is compatible with the external monitor.
3. Update the monitor's driver and your display adapter driver.
4. Check and see if there are any available resolution settings that match the native display resolution of your monitor.
5. Check the refresh rate settings on your computer.
6. Change your display settings on the laptop and/or the external monitor.
7. Adjust the image position and size of the display.
8. Reset the laptop and its display settings.
9. Replace the video cable of the external monitor.