First, try cleaning out the hole where the charger connects to the iPod. Use a wooden or plastic toothpick...or something that won't damage the connector leads. Dirt can collect in there and keep the charger tip and the inner leads from connecting.
My iPod would charge fine on some days, on others it wouldn't fully charge, then it just died. Took it to the guy at apple, he shined a flashlight down in the hole, cleaned the dirt out, and now it's working again.