I have a goodman heat pump system with two nice size return vents. One upstairs and one down. The filter size on the air handeler is 14X20. When I change the filter only approximatly 9X12 of the filter is dirty. Wouldn't this mean that the return size is only 9X12? I am having a problem with low airflow at the vents. A few vents do not have any flow at all just noise from the motor running.