
SOLUTION
Your poor performer doesn't have the skill to perform as you'd like and you've discovered he/she has never been able to perform in the past. The solution is to provide formal training and develop the individual's skill.
Aptitude Problem?
For very complex, challenging jobs, a performer who is motivated to perform and who is given training, practice and feedback still may not be able to perform. Such might indicate an aptitude problem. If you think this might be the case, click the link below:

Solution Explanation
The poor performer may have developed bad habits which will have to be "untrained." Formal training can take many different forms. Perhaps the most common is classroom training, but it may not be the most effective. On-the-job training conducted by an effective coach may prove more efficient and effective. Whatever type of training provided, it should make no assumptions about the skills of the trainee since he/she has never had the training needed to perform the task. After training provide practice and skill testing to ensure the new skills have been adequately learned.