I have a Superior Fireplace "Heatform" fireplace that was installed in 1984. The "Heatilator" type airflow through tubes around the firebox worked well up until a year or so ago. Since then the airflow through the system has decreased to almost nothing. With the two fans on a very small amount of air does come through. It appears that something has plugged the system but I cannot see, not that there is much of the system visible, any obstruction. Any ideas as to what has happened to the air flow system?