Common vision problems can be missed with basic school vision screenings. Sometimes vision problems are misdiagnosed as learning or attention problems and vise-versa.
Children are born with ADHD, it's not acquired. Most of the characteristics of ADHD are similar to many typical behaviors in preschool children, a challenge for diagnosis
Trouble staying focused and sitting still are obvious symptoms of Attention Deficit Disorder, but behaviors are not always the same. Diagnosing ADD includes many factors.