If I'm understanding you, the brake arms are not centered, causing one to move and one not. There is a small adjustment screw on each brake arm to adjust the arm travel so that they both move the same amount when you squeeze the brake lever. I believe you tighten the screw on the arm that's not moving and loosen the screw on the other arm until they both move the same. Here's a video. He starts adjusting the screws I'm talking about at about 2:10.

http://www.youtube.com/watch?v=7oPxuyQ7YKI&feature=related

Here's a link with closeup pics.
http://sheldonbrown.com/canti-direct.html