Air conditioning never turns off and does not completely cool down the house?

Most likely the AC condenser and coil unit are not the right capacity for the size of your house. They are situations were properties has existing AC before any additions and then people never update with a more efficient unit to cover the added areas. For example, let say you have a 2.5 tons AC unit for a 1500 SF and add an additional 700 SF, you must update the AC unit to at least a 3000 ton AC that have smaller capacity than what the property requires will work harder and many times won't pick the temperature you called for. Therefore, the cooling unit will never stop running unless the temperature outside gos low. Have a specialist to determine if you have the right capacity AC for your property. Good luck...!