I recommend that you stop doing what you’re doing immediately.
#1 it’s a heat pump.
#2 you’re using the wrong equipment to test.
#3 your using the wrong technique to evaluate the situation.
I’m not going to get into this because I would have to write a novel about the number of variables that need to be considered to evaluate air temperature of the heat pump heating system. Most of which you would not understand.
If you’re going to do anything, as Wayne indicated you need to be looking at the suction refrigeration line temperature (better yet the compressor discharge line temperature).
You need to know the refrigerant in the system.
Next you need to determine the BTUs per pound of air outdoors. This is the energy available to be absorbed by the HVAC system. If it’s not there, you cannot absorb it. If it’s not there to absorb, you can’t collect it and discharge it into the house.
This is not an electric heater or a gas heater or an oil heater where you can determine the heating capacity of the fuel being utilized.