eli5: Why can’t people with ADHD focus on something even when it’s something they want/need to focus on?

If they want to do or feel a lot of pressure to get the thing done (anxiety can sometimes be used by people with ADHD to get things done) then shouldn’t the dopamine needed already be present?

Edit: I have ADHD and am trying to understand why I sometimes just can’t focus on a task even when I am interested/want to do the task.

Edit 2: I’ve noticed a few people commenting that they think they have ADHD based on the other comments. Please keep in mind that there is a lot of misinformation in the comments and that ADHD is a lot more than issues with focus (and that a range of other disorders/conditions can also cause issues with focus.) If you think you may have ADHD then please seek a formal assessment from an ADHD specialist (professional.) If you’re UK based you can start the assessment process by contacting your local neurodevelopmental department, or by asking your GP to refer you to the neurodevelopmental department.