This usually happens when the Rio’s battery is fully drained, the charger or cable isn’t making proper contact, or the device’s charging pins are obstructed or dirty.
If your device isn’t charging, ensure the cable is secure both in the device, and the wall socket.
The Oysta Rio should sit well in the cradle, held in by magnets. The LED on the cradle changes to red when charging, and green when it's charged fully.