The answer to 'why is my furnace not heating' is that there could be several common issues causing your furnace to not heat properly. Some of the most likely causes include a clogged air filter, a thermostat issue, problems with the pilot light or ignition system, or a mechanical failure in the furnace itself.
If your furnace is not blowing warm air, the first thing to check is the air filter. A dirty, clogged filter can restrict airflow and prevent the furnace from heating effectively. Try replacing the filter and see if that resolves the issue. You should also check that the thermostat is set to the correct temperature and is working properly.