Inspect all forced hot air vents. First visually inspect the vent. Be sure there is nothing obstructing an even flow of air. Move any furniture, rugs or carpet that may be blocking it. Insure that the vents are indeed open. Many vents have small levers to open and close them depending on the season. Open vents to clean.
Vacuum the vents. Use a high-powered vacuum, such as a shop-vac, to clean the vent grill thoroughly. Use a brush attachment to scrub the grill of any pollen or dust build-up.
Clean inside the duct. Remove the grill. Normally there are one or two small screws holding the grill in place. Remove these with the appropriate screwdriver. Then use the hose attachment on the vacuum and clean out the duct as far as you can reach.
Make a final inspection. Use a flashlight to look as far into the duct as possible. Occasionally small toys or other items can fall down the vent and over time will cause dust and grime to build up around them. Remove any objects visible.