I released I'm One! a few weeks ago and saw it soar up the rankings when I announced a free day promotion. Unfortunately, the surge only lasted that one day and I saw exactly one sale since. But I am also guilty of not following up the promotion with more promoting nor utilizing more free days.
One big problem that I think occurred is that I selected the wrong category. Amazon's category selection is not consistent with the way categories are treed on the main site. The first book would have been very deserving of the "Baby-3" sub-category in Children's eBooks, but that was not an option on the KDP dashboard. I am actually still trying to get Amazon to correct this problem and get that book into the right category, but since I published it and launched a free day promotion under different genres, it messed everything up.
Advice: find the categories you want to be in before publishing. Follow the instructions on the site for requesting specific categories. You have to select NON-CLASSIFIABLE and then contact Amazon to determine where you want to put it.