Using 9v battery to power Arduino and separate battery for 4 dc motors

For my first project I have set up a breadboard to turn the 4 wheels of a 4WD Smart Car. The wheels are powered by a separate 4 AA battery pack and controlled through an H-bridge using both sides.
The sketch I wrote to turn the motors forward/reverse works perfectly well when the Ardinuo is powered by a USB connect to a laptop. However when I take the USB connection out and use a 9v battery to power the Arduino Uno the motors do not work at all .... just a gentle humming sound.

Obviously I don't want the Smart Car to operate attached to the laptop. I can't see anything wrong with the code because everything works when the lap top is connected. Also, I believe the layout is correct on the bread board.

So why is the 9 volt battery connected to the Arduino causing problems?

9v batteries are unable to supply much current. Try a few in parallel.