Why do some children have a poor grasp of phonics?

Absolutely. I’d also add that the more you read the better your composition skills.

“Visual system” is an oversimplification of the complex processing involved and I agree that some might confuse problems in this area with those some optometrists propose to treat, e.g., putative issues with binocualrity and visual tracking.

I asked my optometrist about his opnion of visual training for reading/learning problems and he asserted that about half the doctors he knew considered it to be hogwash. It’s also expensive.