If a student struggles in school and is not achieving at grade level, there has to be a reason. More of the same instruction generally does not address the underlying causes.
Most students who struggle in school entered kindergarten behind in foundational skills that empower effective learning and they never caught up. Most schools do a fairly good job of teaching students who have the foundational skills, but do not have the resources and technical knowledge to catch up most students who are behind.
As a result, most students who start behind stay behind.
There are three major categories of skills each student needs to learn how to read proficiently: 1) learned skills, 2) mindset skills, and 3) cognitive processing skills.
Learned Skills for a student entering school include things such as oral speaking skills, print awareness and vocabulary. Mindset Skills include self-esteem, persistence or grit, discipline, and a growth vs. fixed mindset. Another term often used for this type of skill is social emotional learning. Cognitive Processing Skills include memory, visual processing, auditory processing, processing speed, attention, and logic and reasoning.
Most teachers have 20-25 students in their K-2 classrooms. It is very difficult for them to evaluate and address the different needs of each student. Most teachers are required to teach a very rigid curriculum and timetable. They don't have the time to go back and catch up students who are behind, especially if the student needs specialized training to close their skill gap.
Over time, most schools have remediation programs to try to catch up students who are behind. But most do not have a comprehensive process to evaluate and address the major causes of reading difficulties.
The following link is for a detailed paper describing the skill gap and how to address it. Skill Gap Summary