After your positive that your pads and pins are almost new and your disc is straight , watch the deflection of the disc when the brake is applied, you can do this with the bike stationary just squeeze the brake and look at the disc if it moves in or out then your caliper carrier is bent (the part that bolts to the fork leg, that holds the caliper in place) so the brake pads arent allowed to conact the disc parrallel, it doesnt take much to affect braking.If you ride in an area with deep ruts it is possible to bent this part without bending the disc.