Could it be that:
📳 the role has changed and this person is bored?
✝ this team member is experiencing personal issues?
💠 other team members are creating friction for this person?
✳ you have been distracted and not giving this person your attention?
✴ organisational changes have impacted this team member’s mindset?
No one wakes up one day and decides that they are going to stop performing. The key is for you to ASSESS what is happening around this person – BEFORE you even think of discussing their performance issues with them.