After 24 hours, sensor data is considered outdated. There are several reasons for this.
The sensor is too far away from a hub and cannot be synchronised automatically.
The app has not been opened near the sensor for a long time to synchronise data.
the battery is flat
First, press the button. If the sensor does not flash red, the battery is not the cause. Check the distance to a hub. If there is no hub, open the app near the sensor and wait a moment.
If no data is still coming through, restart the sensor by briefly removing the battery.