I found one of my cameras stopped switching to night vision . I brought camera inside and took the back off and noticed it got water in it. I removed the guts and cleaned and dryed everything out put back together. The night vision does work, but only if you trigger it to stay on all time (operate manually). The auto mode no longer works. Probably due to the water. This is a outdoor camera and only 2 months old. It only has the back part that is removable to I put black electrical tape around the seam on all other cameras I have outside to keep water out. All others are fine.
If your night vision is not working, check for moisture and water inside.