Hmm, well, I'm at a loss then. If you've adjusted the tension screws to center the arms and the one still doesn't return when you release the brake lever, the only other suggestion I have is to remove the offending arm from its mount on the frame and make sure there are no burrs on it or the attachment point of the frame that may be causing it to hang up. Also don't forget to put a dab of grease on the attachment point when you reassemble, that may help also. If that doesn't help then you may have to take it to a local bike shop and ask them to figure it out. If you have to do that though, be sure to get them to show you what they did so you can do it again if they come out of adjustment, which they surely will at some point.