Brand new ipad 2 won't charge

Bought an iPad 2 in jan 2012. In Dec 2013 it stopped charging. Tried rebooting, lowering brightness and all the other tips but could not get it to reliably recharge. Took it back to retailer who checked the iPad with 2 other chargers and decided there was a fault with the internal charging unit in the iPad. Under the shop guarantee, they replaced the ipad, but this doesn't charge either? They did not replace the charging cable or plug. It's the original 10w charger. Does anyone have any idea as to why a brand new iPad 2 will not charge with a 10w charger?