Could be the cable, but most likely it's the centering over the rim. Loosen the rear nut, centre the brakes and tighten the nut WITHOUT ROTATING THE BRAKE. Easier to do with Shimano brakes as they have flats on the sides of the centre-bolt to let you hold the brake stationary while tightening the nut. On the Campy, you can rotate the brake slightly towards left side so there's extra room, then tighten the rear nut. You'll see that the tightening action will rotate the the brake towards the right side and centre the brakes evenly. On some brakes, I've had to hold the brake with the right pad all the way touching the rim while tightening the centre-bolt nut.