This is a result of not enough light on the subject. This can occur for several reasons:

solution: Turn on the flash or move closer

Remember, most flashes are best if used within 15 feet, if used further away than that, their effectiveness drops off dramatically.

solution: Turn on the flash, open the lens more if possible, or use backlight setting on camera if so equipped

solution: Adjust camera to proper film speed, shutter, and lens setting

solution: Use a higher speed film, such as 400 or 1000

Autofocus cameras

Many cameras have trouble focusing on black objects, this is because the infrared beam(s) used to focus are absorbed by black, causing the camera to be unable to determine the objects distance.

solution: Try and find a light area to focus on.

This can happen when the focusing beam(s) of the camera are not centered on the subject. This can occur when the subject is off center, or when there is a space between two or more people and the focusing beam(s) is aimed between them. The more focusing beams your autofocus camera has, the less likely this is going to happen.

solution: On most cameras, there is a highlighted area in the viewfinder that shows where the camera is focusing. By centering the box on your subject, and partly depressing the shutter release and holding it, you can focus the camera, and then move the subject off center if desired.

Most cameras have to be 3-5 feet away to focus properly.

solution: Move further away, get a macro lens or close-up lens attachment if your camera will accept one.

This is more relevant at slower shutter speeds.

solution: Use a tripod, or a higher shutter speed if lighting permits.