If the intent is not there... if the Atemi is not real... then uke can and very possibly will ignore it as a non threat and it will either loose effectiveness or become completely ineffective.
Sensei has sometimes demonstrated the difference in reaction of Uke when there is real intent and when there is not. I've felt it myself. Real atemi gets better results and is a better teacher.
Or he could ignore it all together regardless of you intent. He may not care about your intent what-so-ever for a number of reasons. So what do you do then?