This could be happening for a few different reasons. Check the following:
1) Is your device connected to Wi-Fi? You can check your network connection within the device settings menu > Network > Wi-Fi
The outdoor readings are displayed via a Wi-Fi connection, so any network issues may prevent these from being displayed on your device screen.
2) If your device appears to be connected to Wi-Fi but the outdoor data is still not being displayed, it is possible that there maybe an issue with the data source that your device is following. Check which station your device is following within the settings menu > Location > Followed location.
You can check the status of the followed station in the AirVisual app. If the station's 'last updated' time is more than 8 hours ago, then this will have been disabled on your device's screen display until the station refreshes with up-to-date information. This may happen when a government station is undergoing temporary maintenance, for example.
If this is the case, please try following an alternative nearby station with more up-to-date information, until the other data source returns.
Related: see 'My Node/Pro shows 'No recent data' for Outdoor air quality'