It depends on what you are connecting it to. If it is attached to a computer while on, there's not enough amperage to charge it. Some phone chargers have the same issue.
If you are getting the red X with the stock charger, there is an issue and you should contact Samsung.
I've had it momentarily. Unplugging and plugging it back in solved it both times.
Well, it was connected to my PC, and it was charging it, even though the red "X" was there, the funny thing is, that it wasn't there the very first time I plugged it in. It was using the stock cable from Samsung that came with it. It's fine with no red "X" when plugged into the wall socket.
Regardless of what it says, I have charged it through my PC.