If your business card is covered in shadows, scans might not be able to tell the difference between text and shadow.
Try to take pictures of business cards where there is plenty of ambient, indirect light. Natural sunlight and diffuse fluorescent lighting are good at filling a room with bright light without causing too many distracting shadows.
Be careful when taking pictures with direct light sources, since they can not only cause dark, clearly defined shadows, but also cause reflections or glare on your cards, which can severely distort scanning results. Try moving your camera and card around so that no direct shadows or reflections fall on the card when you take the picture. Keep in mind that it's not what you see, but what the camera sees that makes the difference in a good scannable image!