This article "Nudging Schools to Help Students With Learning Disabilities " from New York Times gives a brief overview of what parents can to do navigate the process of getting help from the public school system.
This is particularly useful for parents of children with "unrecognized" disabilities, like sensory processing disorder, which alone doesn't usually qualify for an IEP, or ADHD which might not seem as important to address as a more overtly physical disability.
With strapped budgets, it is not surprising that our struggling public school systems try hard to triage students, using limited funds to serve those with the most pressing needs.
So this article can help parents fight for those limited resources for their kids. Unfortunately, this doesn't solve the bigger problem of too many children being systematically denied services that would brighten their future.