I'm running 4 of these bog cameras right now on my property. Sometimes i get blurry images on all of them, but not usually. One of the cameras takes lots of "nothing" images. It's from the sunlight shining through the trees and casting light/shadows on the ground in front of it I think. My other 3 cameras rarely take "nothing" images. I think it's just where and how I have it set up.
I've been thinking I should make milk jug covers for all my cameras to keep rain off them, just for good practice. With other cheap cameras I've had lots of trouble of water getting inside and corroding out the batteries. These bog cameras have been good for me so far, but I still think it would be good practice to keep the weather off them.