Heat pumps produce less heat to the house as the outdoor air temperature decreases. What you are experiencing is hitting the thermal balance point of your house. You know when you have hit that when not only does your heat pump run all the time to maintain the temperature set point, but it also loses ground. In other words, if your set point is 24°C and the unit runs all the time, defrosting often, but the indoor temperature drops below 24, your heat pump can't keep up with the amount of heat the house is losing to the cold weather. You either need to reduce the amount of heat the building loses when it is that cold, or add more heat with another source.
Psychrometrics: the very foundation of HVAC. A comfort troubleshooter's best friend.