Also, old Sony devices with mini-usb don't like to charge over USB unless it's a port on a computer, and they already have enough charge to power on and be recognized by it. That's why Sony put the barrel jack on them. Try using the barrel jack and see if you get a full charge, or plug into a powered on computer.
I have an in-line USB power meter that is super handy to see if something is drawing power and how much.
EDIT:
Here's the manual as PDF. Jump to the Charging the Reader section.