You could try realigning the caliper, if the brake pads aren't the problem. Your brake might have a dial or something to adjust the position of the brake pads relative to the rotor. This might not work on your brakes, but with hydros you just loosen the mounting (post) bolts a little, hold the brake down and retighten for minor alignment adjustments. If its not that I reckon it's new pads, which could mean new brakes (but take it to the bikeshop first, they'll know a lot more than me).