Anyone who has worked with Ecclesiastes knows, discerning the book's structure is very difficult once you get beyond the prologue and epilogue. Daniel Fredericks might be overstating things a bit, but he does sum up the challenges and frustrations faced by many interpreters.
"It is the goal of every commentator to retain and reveal any and all strains of linear thought in a text. When it comes to Ecclesiastes, this is apparently impossible. A systemic, cumulative pregression in thought which is sustained to a climactic end is not found in the speech. Since the structure is so elusive, commentators have done their best to bind similar material together, stretching literary units to absorb as many verses as possible."
Daniel C. Fredericks and Daniel J. Estes, Ecclesiastes and Song of Songs, Apollos Old Testament Commentary (Downers Grove: InterVarsity), 61-2.