In the novel Things Fall Apart, Achebe educates his readers about the Igbo society but he does not present it as perfect. Why does Achebe not present Igbo society as perfect?
One of the things that Achebe was trying to do in Things Fall Apart was to paint a relatively accurate picture of Igbo society given that those portrayals were so few and far between. In order to do this, he drew on his own experience and that of his own tribe and his own upbringing. In order to present this picture as the foil to the previous portrayal of Africa in stories like Conrad's Heart of Darkness, Achebe had to attempt to be more balanced in the portrayal or else he would essentially be doing the same disservice that Conrad had done in his one-sided representation (even if Achebe would be on the other side of it).
So he tried to tell the story about Igbo culture, both the good and the bad parts of it. And in doing so, he also demonstrated that the heart of so many issues were misunderstandings and an unwillingness on both sides to communicate in such a way as to bridge the gaps of understanding between cultures. Had he presented the Igbo as perfect, Achebe would have failed to provide one of those bridges.