I had this same issue before I started my degree. I just couldn't see what was wrong with my writing. I found that feedback from others helped because it pointed out things that I couldn't see myself. Eventually, I learned that by putting my writing to one side for a day or two, by the time I came back to it I was reading it with fresh eyes, like I was the reader and not the writer. It was easier to tell whether the writing flowed or if there was unnecessary exposition, for example. You're right though, there is an elusive something that some writers just seem to have any the rest of us struggle to find.