Just because the food bowl is empty does not mean that one cat is not experiencing a reduced appetite. As cats have a strong sense of smell they can smell the medication that you hide inside the food. Depending on what the underlying cause of ADR is treatment may be either outpatient or require hospitalization.
Lastly recent vaccination may throw your cat off food for a while due to the shock and trauma that your cat goes through in the process. Your cat is maybe having any issue or a specific disease. However you have to face a situation where your cat is not a craving for food. Just like people your cat may not want to eat if its mouth hurts.