I ran into the same problem on two calls this winter. One unit was about 12 years old and the other was about 3 years old. On both I pulled the motors and cleaned the obstruction from the inside. I used an orifice bit (same size as the port) I also inspected the flu and burners. Both units were burning correctly with the correct gas pressure. The only thing that they had in common was the blown in insulation. The real fluffy white stuff and the installers were not very careful and had gotton some around and on top of the units. The obstruction was pale and crusty, somewhat like the stuff you get around a standing pilot orifice that dirty.
Haven't had a call back yet.