Can't side-lay scan
Low brightness（the red area is not enough to be recognized）
Restart calibration software and adjust the gains，until all of the white dots become red（follow this picture）
High environment light
Light is too much strong surround the scanner,try turn off the light,close curtains，back to the light source.
Dust（the lens or calibration plate was too dirty）
Clean the dust on calibration plate or camera lens.