Micheal
I might be wrong but if the heat pump function of these A/C units is making good heat then they must have sufficient refrigerant to do the job.
It might be low refrigerant but it could also be a problem with reversing valves or some another part of the units the best way to find out would to be have an HVAC tech do a full pressure test .
But before having them looked at I'd lift all the covers and check the condenser and evaporator for dirt and proper air flow.
Hope this helps
Eric