Unfortunately our images (we call them subjects) are not always perfect for classification. The camera system can get blown around in the wind or focus at the wrong point, these circumstances cause the images to be blurred. Sometime cloud comes along and reduces light levels. To combat this we use a higher ISO (since shutter speed and aperture are fixed in our set-up). This can introduce noise, and if it is really dark, too much noise to be of use.
Here are some examples where imaging problems have reduced clarity