Hi
To eliminate a fault with the battery or the usb port on the device, try a different charger (and lead if separate).
If you are using the tablet intensively while it is plugged in charging it is possible that it is using more power than it is storing. In that case, turn it off and leave it charging for 24 hours and see if that helps.
If still no good is purposely a hardware fault in the docs which will need repair (warranty? )
Roger