Some applications (Ruuvi Station, IoTool, etc) require permissions in order to see Ruuvi sensor. Check granted permissions in OS settings.
Check power optimization features
Disable power optimization features in phone settings for Ruuvi Station.
Location is not enabled
Android doesn’t allow Bluetooth scanning if location is not enabled. Services running on Android 10 and higher cannot discover Bluetooth devices unless they have the ACCESS_BACKGROUND_LOCATION permission. More information can be found here https://developer.android.com/guide/topics/connectivity/bluetooth#Permissions
Bluetooth is disabled
Ruuvi sensor uses Bluetooth (BLE) to broadcast data. Check that your Bluetooth connection is enabled. Notice that the Ruuvi sensor will not show up in your mobile device Bluetooth settings because Ruuvi sensors are not connectable Bluetooth devices as they are only broadcasting.
Ruuvi sensor is not powered
First run: remove the resist between the battery and the battery clip. Indicator LED light will blink on Ruuvi sensors when the beacon is powered. Change the battery if the LED indicator continues to be off.
Ruuvi sensor firmware
Check that you are using the correct firmware for your application (Ruuvi default firmware, Eddystone, Espruino). Refer to https://lab.ruuvi.com/dfu for more information regarding firmware.
Ruuvi sensor signal is out of range
Bring Ruuvi sensor closer to your mobile device. Make sure there are no structures between the Ruuvi sensor and your mobile device, which could obstruct the Bluetooth signal.