Here are some possible reasons:
Alert not enabled in sensor Settings
Check that alert is enabled in Sensor Settings.
Alert range too narrow
Make sure that the alert trigger is configured correctly for your needs.
Bluetooth is disabled
Ruuvi Station requires Bluetooth (BLE) connection, check that your Bluetooth connection is enabled.
Check your device permissions
Ruuvi Station requires location permission in order to see Ruuvi sensors.
Notifications are disabled
Ruuvi Station uses notification settings to show alerts. Check your mobile device notification settings.
Sensor is not powered
Indicator LED light will blink on the Ruuvi sensors when the beacon is powered (the LED is visible only when opening the case).
Ruuvi Station can only see sensors, which have the default Ruuvi firmware program.
Sensor signal is out of range
Bring the sensor closer to your mobile device. Make sure there are no structures between the sensor and your mobile device, which could obstruct the Bluetooth signal.
Background scanning is disabled (for Android only)
Ruuvi Station will only listen to Ruuvi sensors when the app is active in the foreground. Enable Background scanning in App Settings to run scans when the app is working the background.