Check the thermostat settings. If the furnace is fan is set to run constantly but the temperature setting isn’t set high enough to call for enough heat, adjust the thermostat. If the thermostat isn’t working properly, replace it.
If the thermostat is set properly and working as it should, check the air furnace air filter. A dirty filter will block air flow through the heat exchanger and air vents in your home. Replace the air filter if it’s dirty.
If these tips don’t help, then a problem with the furnace burners or the blower fan could be causing the heating problems. Have a service technician examine and repair the furnace if you’re not able to restore proper heating.