Check there's no muck in the charging port, blow into it and maybe probe carefully with a pin.
Try other cables.
Found this online (nb says volum up button, not volume down) .:
If you can’t find any problem on the charger, cable, and port, you can try this method. Although this method seems weird, but many users have reported that this method worked for them.
Disconnect the charger from the Samsung tablet.
Hold the power button and the volume up button for 30 seconds.
Keep holding buttons while plugging in charger.
Continue to hold buttons for another 30 seconds.
Release power button only and keep volume button pressed still.