There is a lot more to hearing than just your ear. Signals have to be processed by the brain and that's often where the problem is. A good example are children with sensory integration problems: a child will often go through phases when it will have trouble "hearing", but the actual problem will be with processing. You then have to repeat things several times so that the child "hears" you, but at the same time the child will have no problem with hearing quiet sounds. Kids are especially interesting because as they grow (and with therapy) you can observe as these things change over time.