Your heating system works hard all winter to keep your home warm when it suddenly starts blowing cold air there are several things to check. First, it's important to remember that the furnace is only one part of a central heating system. The air ducts are what deliver the heated air to all areas of your home.
A good place to start when your furnace isn't heating well is the air filter. A dirty furnace filter will prevent sufficient airflow to the furnace, making it difficult for it to heat your house. Depending on the type of filter and the condition of the air in your home, it should be replaced every 1-3 months.
Leaking air ducts are another reason your furnace may not be heating enough. Air ducts can develop cracks and broken seals over time. Inspect the ducts and repair any leaks around seams. If some locations in your home are warm enough but only certain rooms are cold, it could be that your air ducts are not sized properly.