I am using the BB7 on my mountain bike. They are excellent.
Except if your pads are worn out, I am pretty sure your problem is in the cable tensioning. When attaching the cable to the brake unit (caliper), first make sure the pads are as far from the rotor as possible using the red knobs, then make sure to hold the brake unit lever (not the handle lever) tight almost to the point of having the pads touch the rotor. Tighten the cable with enough torque and then adjust the pads to be about 1mm from the rotor.