Chemical tests show lower than normal levels of dopamine in that section of the brain. This blocking effect allows these chemicals to accumulate to more normal levels. And the clinical picture usually confirms that hypothesis. Treatment with stimulants makes most everyone function a little better. But if the diagnosis of ADHD is made and one does not do well with stimulant medications another diagnosis needs to be looked for.
Medication is just one part of the treatment.
If medication is the only treatment then the child will not as well. Not all children with ADHD need medication to get by. Years ago there was a lot done with diet and food additives. When parents spent every waking hour thinking about and protecting their child with special food and constant attention the child often did well. Most of us do not think this form of parenting is wise for the child or the family.