To be successful you need the employee to want to achieve the desired level of performance.
A good way to create that environment is to seek the employee’s involvement and ask them to contribute to the solution.
"We need to quickly address this problem. You’ve managed to achieve the KPI on some occasions. What I’d really like to do is get your ideas on how we can do that consistently".
When people think you are sincere and value their ideas they will be more willing to cooperate and actively work towards resolving the performance problem.