Think about the first time you drove a car. Imagine yourself staring at all of the unfamiliar levers and knobs and pedals, at the panel covered with gages and numbers. Try to recall sitting frozen in the unfamiliar driver's seat, frantically going through all the steps you had to remember just to get the car out of the parking spot. It is hard for most of us to remember how overwhelming it was to process and recall all of those steps: to pay attention to the road, to remember the rules, drive the car, and to follow directions. There was also probably a very nervous and impatient instructor who couldn't seem to cover up her frustration at your clumsiness and your inability to remember all of the steps: for her, driving had become second nature.
Like driving a car, reading and writing are complicated processes; each requires numerous instantaneous and simultaneous steps, steps most of us complete without a second (or even a first) thought. Most of us probably cannot recall what it was once like to learn how to read and write, and certainly a great number of us never had to deal with another level of challenge, a learning disability that made these steps even harder to complete.
For many people with learning differences, the challenge of reading and writing are like having to learn to drive again every single day. The steps it takes to write that sentence or read that paragraph are not internalized, but rather feel like huge barriers between the student and the completion of an assignment. And you, the writing teacher, are like that frustrated instructor who cannot understand why this student cannot simply "drive" (or write, or read, or remember, or apply knowledge . . . ).