It means its battery is low, charge the battery for at least 3 hours and then try to sync it again.
If still face any kind of issue with it then do write us back so that we can help you further.
First of all,Reboot your base station and remove the batteries from that specific camera and wait for at least 15 seconds.
After that reinsert the batteries and once you insert the batteries make sure that blue light should flash(for some seconds)
Now , press and hold the sync button on your Base Station for at least 3 seconds and then release it and after that press and hold sync the button on your camera for 2 seconds and then release it.After that blue light will start flashing on your camera and it means that camera is now syncing with the base station.