Final answer:
If the head is repositioned and a repeat ventilation is unsuccessful, the next most appropriate step is to check for an airway obstruction.
Step-by-step explanation:
If the head is repositioned and a repeat ventilation is unsuccessful, the next most appropriate step is to check for an airway obstruction. It is possible that there is an obstruction preventing air from entering the patient's lungs. By checking for an airway obstruction, you can identify and remove any obstructing object or clear the airway to allow for successful ventilation.