Are the Registers Open or Blocked?
Every furnace technician has a story about a cold room that was cured by opening a register or two. Don't make yourself the star of one of those stories. Rearranging the furniture or shoving a rug aside can also block airflow. And here's one reason you might actually want to close your heat registers.
Are the Dampers Open?
Some ductwork contains dampers to adjust airflow. Look for handles and markings on the ductwork such as 'summer' and 'winter.' Set the damper handle parallel to the duct line for maximum airflow.
Is the Furnace Filter Filthy?
This is the most common cause of heating (and cooling) troubles. Change the filter and the problem usually disappears.