It will always say not charging when plugged into the USB since most USB ports do not put out enough power.
That said, you should not get that message when plugged into a wall outlet. I would, if you can, try another outlet, or swap the charger up and make sure it's working. Also make sure you're not using an iPhone charger. They don't put out enough power as well. Your iPad is best charged with the charger that came with it