Help Nexus S USB charging


I have begun to have problems with my Nexus S, when charging via a USB cable.

This has happened twice in two days now. Once from my Car, which has a USB port for charging devices & once from my work PC which has always been fine before.

I plug the device in, the battery icon has the lightning bolt, which usually indicates it is charging ; the icon also usually 'animates' to show that it is charging, but now its not doing that. The device continues to work whilst plugged in to the USB port, but seems only to be maintaining its current level ( i.e. almost flat battery ). This remains the case even if plugged in all day long.

Plug in to a mains charger & it all works fine.

Have I changed a setting without realising ?