Hi Weilan,
Have you verify the lenses on each of the DFI sensors?
Make sure they are perfectly clean, dry and transparent.
If the issue persist, I suggest you contact your dealer for warranty or else you can always replace the DFI sensor by yourself using these 2 links:
Replacement part: https://www.robotshop.com/en/litter-robot-iii-open-air-dfi-hardware.html
Tutorial: https://www.youtube.com/watch?v=zD3dsJsVVsc