Not all mobile device cameras have high enough resolution to take clear close-up pictures of small text (eg. iPhone 3G, iPod Touch 4, iPad 2). To remedy this, some devices have accessories that can enhance image quality.
Unfortunately, the application is very sensitive to blurry images even by a slight camera shake or distant photo. Try holding the camera steady or balancing it against a stable object. (even try a tripod)
Make sure the card is well illuminated, but also avoid any direct light to prevent glare or shadows. Strong reflections, glare, or heavy shadows can heavily distort the targeted text scan area.