Grammar checks often don't understand more complex sentence structures. A strong thesis helps organize your thoughts. Write an outline, if that helps. Tell them what you' ve told them. This means you need to have a good introduction and a good conclusion.
Punctuation can be a severe problem.
Comma usage is crucial to good writing. Proofread your essay carefully and review a basic grammar textbook if you have questions. Reading aloud may help you find extraneous or missing punctuation. If you receive consistently negative evaluations of your writing, you might turn to the Learning Skills Center and the workshops and tutoring they have available. The most common problem with an essay is that it lacks a thesis.