You might be running into the default maximum current, which is set to a low-power device of 100mA. The device, your end of the cable, is suppose to send a USB request to become a high-power device of 500mA (which I don't know how to do). Some USB interfaces will supply a lot more current, but don't expect it from all USB interfaces. If you need a bunch of current, then plug into a USB battery charger, because many will source up to 1 Amp, and the new ones that support iPad will source up to 2.1 Amp.