Tickets assigning to wrong agent upon customer response.

I am hoping that someone might know what is going on with my tickets. I have one agent for whom, when a customer responds to a ticket they've solved, the response is being assigned to another agent. For example my agent has responded to a customer issue and solved the tickets. Normally if this customer then responds this will go back to the agent who solved the ticket but for this agent it goes to someone else. I have checked the setup for the agent and from what I can see nothing is different to any of the others. 

 

Any ideas ?