Why do people with adhd have trouble focusing?

The brain's own : neurochemicals dopamine & norepiphrine don't circulate long enough in the synapses, the spaces between neurons, in the areas of the brain that inhibit behavior & sustain attention in people with ADHD. Stimulant medications prevent their rapid re-uptake by the neurons that release them or rapid uptake by neurons that receive them. Non-stimulants keep norepinephrine circulating in a different way.