Motor Shield and USB Board compatibility

Hi, don't know if i must put this here or on the network sub-forum, but here i go anyway :P

I'm trying to move a small robot with the wiimote library (using a wii controller). For this purpose i'm using Arduino UNO, motor shield expansion board L293D, Keyes USB shield and a bluetooth dongle.

This build works fine when i'm using the USB power supply, but when i try to move it only with the bateries (9V for Arduino, 9V for the motor shield to use with the DC motors) it get's crazy and then stop any type of movement.

I was thinking about a compatibility problem between shield, any tips?

Thanks for the help.

What type of batteries are you using? As you said it works fine on Usb, so how it could be a compatibility problem ? !!! :wink:

Hi I am in such a situation: 2 servos and nothîg but usb cable to move them. How did you solbe please ?