Makes a big difference whether they are working under you as a Principal Broker/Supervising Broker or if it's an agent from another Real Estate agency. If it's an in-house situation you do a face to face sit down and discuss the issues or problems and how they will be resolved. If it's an agent from another Real Estate Agency I would first try talking to the agent and if that does not resolve the issue I would go directly to that agents supervising Broker.