Diagnosis
The infrared (IR) LEDs around the camera lens aren't providing enough light, or there's an obstruction. They should glow with a faint red dim light in the dark.
How to Fix It
First, ensure the room is actually dark. The night vision won't engage properly if there's any ambient light like a nightlight, LED clock, or streetlight coming in. Make the room as dark as possible. Check that the camera's IR lens cover (the dark plastic circle) is clean and unobstructed.
If That Didn't Work
The camera might be stuck in day mode. Cover the front of the camera completely with your hand for 10 seconds, then uncover it. You should hear a faint click as the IR filter engages. Look closely at the ring around the lens in the dark—you should see the faint red glow of the IR LEDs.
Heads Up
If some IR LEDs are lit and others aren't, or there's no red glow at all in total darkness, the IR board may be faulty. Also, remember night vision is monochrome (black and white/green); it will always be grainier than the color day video.