My kid was running for a few minutes. Why was there no new data found?
The sensor tracks activity in 15-minute cycles. After synchronizing the sensor, you need to wait for 15 minutes before you can sync it again and read the activity data from the sensor. The limit has been set to optimize the battery life.
Also to see the activity in the app, you always need to transfer the data from the sensor to the app by syncing the sensor. The app doesn't do it automatically.