The cable is not pulled tight enough or is binding in the cable housing. Remove the cable and pull it completely out of the housing. Take a can of teflon spray lube that has one of the little spray tubes and spay it into the end of each of the housings. Reinstall the brake cable and make sure that it is pulled tight prior to attaching it to the brake and adjusting it.
There is one other thing that you might want to check first, make sure that all of the cable housings are seated in their respective receptacles. If they are not properly seated, you will get the symptoms that you are describing.