From AZVisionTherapy.com :
Functional vision problems in children and adults often go undetected by doctors and vision screenings, so those who need help aren’t always getting it. Most school screenings only measure how clearly a child sees objects in the distance. So even if your child has 20/20 eyesight, they could still have a functional vision problem that affects their learning ability. In fact, children with vision-related learning problems often have 20/20 eyesight (visionandlearning.org).
Undiagnosed vision problems in children can manifest as behavioral problems, falling behind in school and homework battles.