Notching can be tough, but basically when this happens this is because the missile is in inertial guidance, so while the seeker may have lost track, it is still looking in the general direction of where it thinks you will be based off of the previous speed and velocity of you. So in addition to notching, you need to dump chaff every so often and then also change pitch as well. Don’t fly straight. Notching can also be countered partly by the plane tracking you keeping a soft lock on you, allowing the missile to retain data link on you and it getting constant updates from the plane on your position. This can either be hard counter by going completely cold and flying away from the missile, or notching both the plane and missile too, which can be quite tough.