Not original, too vanilla, or the material is underdeveloped. It's already been said better by others. The text may have some excellent points but it needs a new angle. A little drama and humor goes a long way in keeping a reader's interest. Also, less is not more.
Text needs anecdotes, examples, and illustrations to enliven the writing.
Not convincing or unclear. Poorly developed thesis. The text is full of faulty logic or incomplete transitions and explanations. Text needs compelling examples to back up concepts. Go here to see some things a writer can do to avoid turning their reader off.