Make sure the sensor is turned on (!) and that its battery has enough charge. When the sensor is on several LEDs are lit, including the leftmost (green) one indicating power. Plug it in using a micro USB cable, and the rightmost (red) LED should light, indicating that it is charging. That LED goes out when the sensor is fully charged. The sensor info available in the Settings menu of the apps includes the charge level. If it is less than 35% the sensor may not work well.
If the rightmost, red, LED is flashing it means that the battery charge is very low. You should charge it, and not do any scanning until it is over 35%. You can see the charge level in the app: Tap the side menu icon (three bars near the top right) and choose Sensor from the menu. The charge level is near the bottom of the displayed information. You may need to scroll down.