I think there can be different reasons for a lower PSI score. my older kid has ADHD and you can see obvious slow processing, in that if you ask her 8X7 she will typically stare blankly at you for several seconds before answering. Her writing is very slow, it took her ages to do her cursive writing packet for school. PSI requires copying symbols from a key for the coding section so a kid who writes slowly isn't necessarily going to do well. My younger kid has fine motor issues and even though he is mentally quick (at least, quicker than his sibling), he would not do well on any test that is timed and requires motor ability to do well. Some kids may also have visual tracking issues affecting the score.
You can easily test this by putting a pencil about a foot in front of his face, moving it slowly around, and asking if he ever sees a double image, or two pencils tracking it with his eyes. (I know someone is going to say that's not an accurate test, which is probably true, but it would flag issues that are obvious and severe, I think).
