New iPad not charging from iMac usb port

After I synced the new iPad with my old iPad 1 settings from my iMac, the tablet showed 'not charging' at the top right. I re-plugged the 3rd party cable. The new iPad showed as charging, but there is no progress in the battery filling up.I have used the same cable on my iPad 2 for 8 months. However, new iPad was failing to charge from iMac usb port. Then I used the wall charger, and it worked