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 threshold is configured correctly for your needs.
Bluetooth is disabled
Ruuvi Station requires Bluetooth (BLE) connection when using Bluetooth connectivity to listen to Ruuvi sensors. Check that Bluetooth is enabled on your device.
Check your device permissions
Ruuvi Station requires Bluetooth connection and nearby devices permission (Android) in order to see Ruuvi sensors.
Notifications are disabled
Ruuvi Station requires permissions to show alert notifications. Check permissions and settings related to notifications on your mobile device.
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 or Ruuvi Gateway router, depending on which one is used to collect data from your Ruuvi sensor. Make sure there are no structures between the sensor and your mobile device or Ruuvi Gateway, that could obstruct the Bluetooth signal.
Ruuvi Gateway is offline
Make sure that your Ruuvi Gateway router is powered and configured correctly. Ruuvi Gateway requires Wi-Fi or ethernet connection to access the internet and relay sensor data to cloud.
Ruuvi account has Ruuvi Cloud Free subscription or hasn’t updated yet
Ruuvi Cloud Free subscription plan doesn’t include push notification alerts. Furthermore, it’s possible that your update interval hasn’t yet triggered the alert. Requires Ruuvi Gateway router. Read more about Ruuvi Cloud subscription plans and features here.
Background scanning is disabled
Ruuvi Station will listen to Ruuvi sensors when the app is active in the background and Background scanning in App Settings is enabled. For Background scanning to work on iOS, you’ll also need to pair each sensor on Sensor Settings page.
Depending on the model of your mobile device, recording sensor data in the background can be more reliable when the phone is connected to a charger and when any form of power management settings are disabled for the Ruuvi Station app in the operating system settings (Android).