There are people who are diagnosed with ADHD. These people usually find it harder to learn than usual. They normally do not concentrate on the mistakes that they have done in the past. This means that in exams if they see some questions that are similar to what they answered before, they may still input the wrong answer even if they know that it is wrong.
At the same time, they find it hard to create goals that will allow them to think about their future. They live in what is present. In fact, they live permanently in the present. They cannot look back or think ahead. This explains why they may find it harder to learn.