If you give a rescue breath to an unresponsive adult and the chest does not rise, the next step is to ensure that the airway is not obstructed. You should:
- Reposition the head to ensure the airway is open (tilting the head back slightly and lifting the chin).
- Check for any visible obstruction in the mouth.
- Attempt another rescue breath.
If the chest still does not rise after repositioning and checking for obstructions, you should call for emergency assistance and begin chest compressions as part of CPR.