I'm having the exact same problems as the person/people in this post: http://www.ez-robot.com/Community/Forum/posts.aspx?threadId=2274
The motor that is unresponsive is the one that has a default value of "1" in Ezbuilder. The other motor works perfectly. When the value is adjusted, it does not make a difference and the other motor does not move. When the left motor is adjusted, it does changer. When the right motor is adjusted it still does not make the motor move.
I rewired everything just to make sure, but still only the one motor works from the EZbuilder. I do believe this is the same bug here again.