The usual PC has 500 mA of power. The wall unit is usually double that and when the iPad 3 is on, it may take almost all of the 500 mA leaving nothing for charging. The workaround we use is to turn off the iPad and leave it connected so it will slowly charge.
I just started having this problem when I updated to the newest IOS from Apple and I am trying to find out if it is wide spread or I have a problem with my IPad.
I can charge my device using an electrical outlet but if I choose to use my windows based machine I get a notice by the battery indicator saying "Not Charging"
I have had my device for the last 7 months and I have now notice after the IOS update if I plug in my iPad using the USB cable it will not charge but iTunes will still recognize that I have plugged in my iPad and I can sync or download or do what I want to but charge the unit. Apple has been a BIG flop in this matter saying that there is nothing wrong with my iPad. If somebody has an answer I would be grateful