I would guess that you are not getting the syringe back far enough in the throat. On some horses I pull the tongue out to one side and then squirt it back in the throat. But, most of the time you can insert the syringe into the side of the mouth and get it back far over the tongue and then squirt—if you are far enough back they have to swallow. Keep holding the head up and rub the throat area between the cheeks to encourage the horse to swallow.