It's embarrassing to admit, but a similar problem happened to me once and my goof caused me to misdiagnose a battery charging problem. At a glance, I mistook the USB cord plugged into the back of the charger base for the power cord. USB is handy interface, but not very useful for charging battery packs.

Make sure you're not dealing with an N.P.I. error (Not Plugged In!). Based on your description, it's unlikely you have the same situation, but trust me. Give it a second look just to be certain.