Light and angle
Move to brighter light or turn on the flash button in the scan screen. Avoid glare: tilt the card slightly so reflections move away from the barcode.
Keep about 15 centimeters of distance. Too close and the camera cannot focus; too far and the lines blur together.
When the barcode is damaged
A scratched or faded barcode may never scan. In that case add the card manually: the number printed under the barcode is all you need.