So, what happened?

Yes, it can be a problem with current and voltage requirements.
The max voltage from a USB port is 5 volts.  There are voltage drops in the H bridge.
What about current?  How much current does each motor take?

Turn the motors on with the program and measure the voltage across the motor leads.  
 

How much current does each motor take?

If the motors work directly off the battery but they don't work when using the USB, then most likely the USB can't supply enough power to run the motors.

Post a picture of you setup with the the wires clearly visible.  Something may be wrong there.

Why don't you try the code right out of an Instructable to see if it works.

Here is the link.

http://www.instructables.com/id/Arduino-Motor-Shield-Tutorial/step4/One-Motor/

Do you mean webmaster@arduino.cc ?

FYI, more people could help you if you posted a readable version of your schematic (pdf) and also legible pictures of your setup/board.

You need decoupling capacitors, thicker power and ground traces and mounting holes at the corners.

You need to post the code you are using and the schematic of your hardware before someone can help you.

Recheck the 22pf capacitors.  They look kinda large to be only 22pf.

Yeah it's a trend that's been going on for several years, I remember hearing about it in 2006 or 7...
Copper has a different coefficient of expansion than gold and thus when driven real hard tended to part at the ultrasonic weld to the breakout pad on the die...
Was a lot of talk about ti... Seems to me that it was National Semi. that started the research...
God Bless Bob Pease... Wherever he is... Bob and Bob (Robert Widlar... inventor of the Band Gap device) WERE National Semiconductor and National folded right about the time that Bob Pease died...

Bob

Ironically, Bob Pease died in a car accident after attending a memorial service for another famous analog designer, Jim Williams.

Hi Bob

Would that be a simplified path to standalone if you use a Goldilocks for the prototype?  If so, any idea what kind of cost you're going to be able to supply these for?

Cheers ! Geoff

Goldilocks is not due out until July.  It is $45 and an additional $10 to ship it here to the US.

Crossroads can turn the board around in far less time at a lower price and it will have the advantage of being able to be plugged into a present design.
They are 2 different approaches.

I like the idea.

Also post your code.

Truthfully, there are maybe 100 items that you would need to look into to get the highest reliability.  Such as:
What temperature range/humidity range will it have to operate at?  This will determine the components you need to choose.
What are the power supply requirements?  Are batteries involved?  Batteries turn this into a more difficult solution.

There are things that you can do to improve reliability:
One board.  The fewer connections, the more reliable. 
Double connections going off the board.
Good connectors. 
Quality parts.
Test it before you send it out.  Test it every which way.  Low voltage/High voltage.  Low Temp/High Temp.  Vibration. 

Good luck!

why do you ask? any of my statements not sitting right with you? if so let me know as i enjoy nothing more than setting straight those who think they know more than they actually do. i can usually go into much more detail and happy for the chance to do so.

anyway, no personal info. the LAST thing i need is having some of these internet characters flollowing me home.

That is what I figured.  You don't have an EE degree.  It is obvious.