Since decisions to remediate the problem will depend on the diagnosis, accurate assessment is crucial. Each of us has strengths and weaknesses that determine if we can learn or perform a task. Poor aptitude for a task could mean that the person could never learn how to do it, even with all the supports in the world. Assessing aptitude is very difficult. Even the simplest responsibilities require skills.
Skills differ from aptitudes in that they can be learned, up to the limits imposed by aptitude.
If the answer is no, then it could be a skill problem. A person must understand the nature of the task, and what is expected. If this clear communication is lacking, no amount of skill or motivation will bring about effective performance. Performance management is the common means for conveying understanding of the task. The best way to assess an employee's understanding is to ask questions within a coaching environment.