Brake Disc Pads problems of the 2007 BMW 750

It has been happening from 2009 which was not a big concern after I bought the veh. As of now the problem has worsened I happens when I am parking the ( parallel parking ) less then 10 miles per hour brake pedal become stiff when depress on it the car would not stop sometime I would bump the in front of me or behind me. The scary part is when I am effecting a broken u turn in a tight street ,I would step on the beak to avoid hitting in front and back of me . The brake pedal would stiffen even if pushed all the way down to the floor. I purshased the car with 13000 miles and now the car has only 73000 miles I change my rotors on time brake pads on time once the car is going straight the brake works please ,I don' t know what to do. Please let me if there is any recall on > I think this is my last BMW ,no more.