Often an employee knows how to perform correctly, the process is good, and all resources are available, but for one reason or another, chooses not to do so, which normally means it is a motivational issue. While many jobs have problems that are inherent to the position, it is the problems that are inherent to the person that normally cause us to loose focus from our main task of getting results. These motivational problems could arrive from family pressures, personality conflicts, a lack of understanding on how the behavior affects other people or process, etc.
When something breaks the psychological contract between the employee and the organization, the leader must find out what the exact problem is by looking beyond the symptoms, finding a solution, focusing on the problem, and then implementing a plan of action. One of the worst situations that a leader can get into is to get the facts wrong.
Start by collecting and documenting what the employee is not doing or should be doing, such as tasks, special projects, reports, etc. Try to observe the employee performing the task. Also, do not make it a witch hunt, but rather observe and record what the employee is not doing to standards. Check past performance appraisals, previous managers, or other leaders the employee might have worked with. Try to find out if it a pattern or something new.
Once you know the problem, then work with the employee to solve it. Most employees want to do a good job. It is in your best interest to work with the employee as long as the business needs are met and it is within the bonds of the organization to do so.