One factor is native language. Young children who speak two languages can often not read or write in either language. If they don't know all the words in English, being able to read or write them is going to be a bigger challenge than just learning how to read would be.
Vision problems are other possible issues affecting the way children read. When our oldest daughter started kindergarten she was put in Title 1 reading because she was considered behind in reading. By the end of the year, her reading level was above grade level, but she was a slow reader. It was not until her junior year in high school that a vision therapist discovered a previously undiagnosed tracking condition. Her eyes tracked different lines and different words at the same time. Her left eye was tracking one part of her reading and her right eye was tracking a different part. He brain had adapted, but there are perhaps students whose brains don't adapt as hers did.
One factor that certainly affects children's ability to read is their physical development. Studies have shown that boys develop physically later than girls, and this condition affects the ability of the eye lens to focus on a page. Many doctors and educators feel that boys' behavior problems in school are due to their initial inability to focus upon the page and learn to read. Because of this physical condition, they, then, are left behind and become problems in nd out of the classroom.
One plan that has been suggested is starting schools at age seven, rather than six years old. But, parentes object. Other ideas include exercises to help develope the eye.