A registered nurse (RN) is overseeing a nursing assistant (NA) on a medical-surgical unit. The RN has delegated the task of taking vital signs to the NA for a patient recovering from surgery. During the shift, the RN notices that the NA is not following up on abnormal vital signs reported by another NA from a different patient. The RN must decide the best course of action to ensure patient safety and effective care. Which action should the RN take next?