blame apple, they changed the charing circuit in ipods at some point so older ipods only charge with older chargers / cables and newer ipods with newer cables / chargers. Basically they charge on a different wire in the cables now
my iphone wouldnt charge so I bought an adapter that allowed both charging wires to charge, so you can charge old and new ipods with it.
i cant even remember what it was called now, ill try and remember to check the name of it when im in the car and post back