This is a common problem for me.
In my experience many devices (with USB charging) can't get enough power even from desktop computer's USB port.
The amount of attached USB devices in the USB-hub can affect to this. For example, charging might start working after you remove some other USB device from the computer and try again. Every device consumes power and the output of USB port is very limited
(According to USB 2.0 specs: 500mA at 5V for one port which is usually shared with many devices).