Lithium-ion batteries have a limited number of charge cycles, after which point they become less adept at retaining a charge, resulting in the device having to be constantly plugged in.
Unfortunately, replacement batteries are not available for the Yoyo (S). As the access point for the device is located behind the Marton Mills fabric, it would not be possible to replace them without first damaging the fabric.
However, if the battery has failed, you will still be able to use the charger with the unit and play from the mains.