I've found if the wattage is too high and the airflow is too low, the coil overheats and says, "fill pod" despite there being enough liquid.
All fill pod means is the coil got too hot because a) too high wattage and not enough airflow or b) there really isn't enough liquid, causing the coil to run too hot. You can experiment with the wattage and airflow until the message stops popping up.