When the furnace (forced air gas) is on, warm air blows from all the vents. When the central air conditioner is on in the summer, the vent in one of the bedrooms does not blow cool air. Why?

Make sure your a/c return filter is clean or replace with a new one, then go out side to your a/c and take a water hose with the unit off and wash the a/c coil out good, you can get some alum bright cleaner at wal mart or lowe,s spray that on the coil then wash with water hose DO NOT put heavy water pressure on coil as you will bend them.