You could have an attention deficit brought on by spreading your attention too thin for a long period of time. You're reading fine, but you aren't absorbing/comprehending the information because that requires singular focus. Today's world with all of its distractions trains us to do the opposite - multitask with our attention on multiple things at once.

The solution? It seems to me that the opposite of spreading your attention in every direction is meditation. Sitting quietly and silencing your mind. Perhaps if you can learn and become comfortable in that state of mind, you can evoke it to some degree while you're reading to help achieve focus.