First I would be monitoring all of my auditors and have monthly debriefs. If I have an under performing auditor it is my job as a manager to work with them to find out where the problems are. I would tell him or her that we are in this together and if my auditor looks bad our entire team looks bad. I would have sessions where the entire team puts issues, concerns, suggestions, best practices on the table so that we are all on the same page. Everyone is capable of doing a good job, however as a manager it is my job to make sure I am giving my employees all the tools they need to succeed. And the most important part of that is to be aware of your auditors skill set so that you can help them enhance those skills as needed.