You may find an employee is struggling in their role and they need support. As their manager you need to Identify areas where you have concerns and provide evidence. When you speak to them give them examples and then listen to their response. Do not jump in with what you believe is the solution or reason allow them to identify why and how to resolve it.
After listening to them you can identify areas where they may need training or assistance. It maybe a case they are overwhelmed with the amount of work. There could be an external problem that you are not aware of. Do not pre-empt the answer. Work with them for a realistic plan with identifiable steps for them to work through. These should be documented with clear actions and when you expect results, as well as when you will next meet. It must be clear for both parties.