Are you using the supplied charger and plugging into a wall socket or are you connecting to a USB port on a computer. Some USB ports do not supply sufficient power to do more than trickle charge the iPad when it is in sleep mode.
Are you charging through wall socket or through the USB port on your PC. If through the PC, then fyi, the rate of charging through USB connection from PC is slower than wall socket charging for all devices. If you use the iPad, you will find the rate very slow. Like mine does not charge at all even if I don't do anything. It says 'not charging'.