There are two parts to the night vision. Make sure night vision is on by tapping the crescent moon icon at the top of the Live Stream. First check the IR LEDs by looking at the face of the camera. The LEDs will appear as faint red dots around the lens.
Next switch night vision on and off and see if you hear the IR filter clicking in and out. If not, that may mean it’s stuck. You can try to move it manually by holding a strong magnet up against various sides of the camera.
If either of these is not working, then I’d try a factory reset per the troubleshooting guide found here:
If that fails, you will need to contact support for a possible replacement.