Unlike speech, reading came relatively late in human evolution so some of us lack the neural software needed to convert fast moving visual symbols (words) into meaning. Reading has to be taught and mastered gradually, building from simple tasks like learning letters and sounds to higher level tasks, such as comprehension.
Picturing the brain as an iceberg, the tip is the part called working memory, where conscious learning takes place. The remaining, larger part is the subconscious where mastered skills are filed. Once mastered, the brain stores these skills in the subconscious and automatically retrieves them whenever needed. This leaves working memory free to concentrate on higher-level reading skills like comprehension. Because working memory capacity is limited, our brain uses it efficiently by limiting the time any particular material stays. Skilled readers have time to master reading material before it is shifted out of working memory but impaired readers (like people with dyslexia) process reading material more slowly so it is moved out of working memory into the subconscious in bits and pieces.
This slow processing can mean that material is not understood, mastered or memorised and cannot be recalled automatically. Consequently, it has to be retrieved and used consciously in working memory, causing it to become overloaded. Overloading the working memory with low level skills means there is insufficient capacity left to master higher-level skills like comprehension.
Traditional methods of support and tutoring repeatedly reintroduces the same material into working memory and this causes the brain to divert attention away from working memory, shutting down motivation and the ability to remember. Repeating the cycle over and over can make students less receptive to learning and, over time, the gap in reading skills between what is expected and what is achieved becomes greater.