Charging Ipod Nano

I bought two USB chargers: one charges from the cigarettes plugs, the other from the household outlet. No matter what I did, I couldn’t get the charging sign on my Nano while I have no problem charging it from the USB port of my computer. I really doubt that both of them are defective (LED on the chargers are on). I wonder whether any one know any tricks to make them work or whether anyone experience similar problems.
Thanks in advance