Diana ult miss

Had it happen few times now. The ult would not register dmg if i move too fast after it. There was no cc or mitigation. I q, ult, the champ is at like 20%hp, i ult again but move fast away since it's inside tower range for example, and the champ lives like i never hit it with same hp. Same thing when i ult and they flash away. I dash to where they were, not where they flashed. I've had this last one happen only once. The champ also didnt take any dmg in that occasion.