Simply replace the unfilled areas such as "" and "[date]" with your information.
I would like to discuss some issues that [name of manager/supervisor] has brought to my attention, concerning your performance in the last few months. These are some of the issues that have been related to me:
1) [description of issue #1]
2) [description of issue #2]
3) [description of issue #3]
I know how hard you work, and you have proven to me that you are a great team player; I greatly value you as a member of our organization and would like to help you get back on track.
I have made some changes in my schedule so that we can meet together and find a way for you to overcome some of the issues that you're having, and get you back on track.
Please get back to me with confirmation of our scheduled meeting.