If the house uses wooden doors, poorly fitted or old ones might result in cold draughts that pass through the gaps.
A poorly insulated roof or a totally uninstalled one can account for up to 90% of the heat loss that happens in the house.
At night, radiant heat can be lost very easily through windows and glass surfaces. If you have many single-glazed windows that you don't cover by curtains at night, your house might lose a lot of heat. See also how to stay warm in winter.
For your house to be warm in winter, the radiator's output must be suitable for the size of the house. If the radiator output is weak, the house might feel cold.
A significant amount of heat can be lost through a chimney. If you don't use yours, consider using a chimney balloon to block the air passage.
Letterboxes can result in many mini-draughts. If your letterbox is not properly secured, your house might get colder as a result of these draughts. See why some people hate winter.
If you place furniture near the radiator, the furniture will absorb a lot of heat from the radiator, thus resulting in a colder room.
When you leave the doors of the rooms you don't use open, you allow cold air to move through the house. Closing the doors of unused rooms can help make the house warmer.
If there is any leakage in the air ducts that pass warm air inside your house, the house might get colder. Make sure you check the air ducts well. See why is it getting colder each year.
Old windows can bring a lot of air in and can result in a cold house. Old windows might not be fitted tightly and so might result in making your rooms cold.
Any problems with wall insulation might lead to leakage of air and so make your house colder. Make sure to check the walls well for cold spots.
Uninsulated floors can result in about 10% of heat loss in the house. Make sure the floors of all rooms are properly insulated so that the house doesn't get cold in winter.