If you could describe the problem or even take a picture, that would make things easier. Are you using the standard/unaltered Downtube wheels? did you check the cables to see if they slided?
Sometimes the problem is not on the brakes themselves: poorly assembled brakes can end up touching where they shouldn't or something else I don't even know.
One way or another, someone here (more experienced and qualified) will be able to help you soon.