The main difference between a convincing, insightful interpretation or argument and a weak interpretation or argument often is the amount of evidence than the writer uses. This means that, for every claim you present, you need to support it with at least several different pieces of specific evidence. Often, students will present potentially insightful comments, but the comments are not supported or developed with specific evidence. When you come up with an insightful idea, you are most likely basing that idea on some specific facts. To present your interpretation or argument well, you need to state your interpretation and then explain the facts that have led you to this conclusion.
Effective organization is also important here.
As stated above, body paragraphs generally have between six and twelve sentences each, so, if you find that your body paragraphs are shorter than six sentences each, then you might not be developing your ideas in much depth. Often, when a student has trouble reaching the required minimum length for an essay, the problem is the lack of sufficient supporting evidence. In an interpretation or argument, you are trying to explain and prove something about your subject, so you need to use plenty of specific evidence as support. A good approach to supporting an interpretation or argument is dividing your interpretation or argument into a few significant and related claims and then supporting each claim thoroughly in one body paragraph. Sometimes a student will write a well-organized essay, but the essay does not shed much light on the subject.