First, read your essay out loud to yourself. It sounds strange, but it catches a lot of run-on sentences, wordiness, strange transitions, etc. If what you write doesn’t make sense out loud, change it!
If you are told your essay is vague or unclear, look at the topic sentences of your body paragraphs and see if they match up with your thesis.
Your essay may suffer from organisation issues, and you might have to alter key sentences to get it back on track.