How are you charging? from the mains or via the usb on your pc?
If it's your pc then have you got any other usb devices connected?
If it's not the USB is the way it's charging different? for example is the display always on while it's charging?
I remember when the screen saver application came out, it could discharge the phone faster than it could charge.
Twitter me @ashwarner
Windows Phone 7 Expert.
Windows Phone Business Specialist