"The fastest way to charge your iPad is with the included 10W USB Power Adapter.
iPad will also charge, although more slowly, when attached to a computer with a high-power USB port (many recent Mac computers) or with an iPhone Power Adapter.
When attached to a computer via a standard USB port (most PCs or older Mac computers) iPad will charge, but only when it's in sleep mode.
Make sure your computer is on while charging iPad via USB. If iPad is connected to a computer that’s turned off or is in sleep or standby mode, the iPad battery will continue to drain.
Also, see here...
Ipad Battery About Best Practice
Check what else is plugged in. I had the same issue this week with my ipad2 plugged into the back of my iMac. Turns out it was the steelseries gaming mouse I had plugged into my keyboard. Moved it to the back of my iMac, bam, lighting bolt. Moved mouse back to USB port on keyboard, iPad back to not charging.
iMac 2.4 GHz core 2 duo. (iMac7,1)