Poor insulation and air leaks can allow the air that your air conditioner has worked so hard to cool to escape, making your house feel hot even when the air conditioner is running. The tracking of temperatures by your thermostat can be hampered by humidity, which also traps heat. The "why is my house so hot" phenomenon may be caused by blocked vents, which prevent the cooled air from reaching other parts of the house, leaving them hot and uncomfortable.