My iPod won't charge

We have two IPOD NANO's. Both have low battery. I plug in the USB cable to my computer and the other end to the NANO as I always do. The computer brings up ITUNES and it does it's syncing. ..and it says it's OK to disconnect. Then the IPOD gives a message "low battery plug into a power source". Both IPODS say the same thing. If I wait a little bit (ie: 3 minutes) it starts doing the SYNC again repeatedly. I have tried this MANY times all with the same result. The HP computer is plugged in to a wall outlet. I am using the IPOD NANO's USB power cord that came with the device when I bought it. I was able to charge one of the IPOD's last week without any problem. It seems very strange that both of our IPOD NANO's would do the same thing at the same time. Could something be wrong with the USB power cord?