1) Check whether the radar is blocked by any foreign object, such as scrap, toy, and remove it if any and then try again.
2) Check whether there is any foreign matter inside radar container, such as scrap, trash, and remove it if any and then try again.
3) Check whether there is too much trash on the floor which interferes scanning of radar, and transfer the equipment to an empty area and try again.
4) Restart the equipment and try again.