If you have a sensor that occasionally goes offline, but usually comes back online, the issue is probably caused by interference or distance. To stop this happening, try to avoid placing other wifi or bluetooth devices (such as smart TVs or speakers like Sonos, computers or wifi routers) between your hub and sensors.
If the sensor in question is further from the hub than the other sensors, the issue is probably related to distance. For help with this, just give us a call on 1300-810-006 or email support@billycare.com.