Try re-topping off the coolant.
First free up some internal volume/space by “burping” your vehicles cooling system aka getting the air bubbles that are trapped deep within pocketed traps making your cooling system run sporadic and overall inefficient.
You just remove the pressurized radiator cap when the car is completely cold, such as first thing in the morning after the car hasn’t been turned on since the day before. Remove the radiator cap, put your coolant burping funnel inside and somehow seal it at the joining of the two, add some identical color/type and preferably brand of coolant specified for your exact model of vehicle, and just fill it up like 1/4 the way, just enough to cover the opening so no vent to the open air exists.
Turn on your car and blast the heater while making sure the “A/C” button is on, yes while blasting it at max fan speed and with the heater engaged or max heat. Let it get up to temperature, expect 15-30 minutes of your car on, so if doing this in a regular homes garage, please make sure your garage door is open and there is an excess of free air and air flow.
Repeat the above in how you see fit but get creative with figuring out how to angle your car in a downwards slope or an incline to help remove awkwardly trapped (usually huge in size) air bubbles that could definitely cause a heater and or an air conditioner to not perform its best or sometimes not at all.
I am not saying this is the guaranteed resolution for this problem, but I do believe it is at least a 50% chance that it is. Worth looking into and or worth checking it out to research this “coolant burping procedure.
There are other possibilities but this to me is the one that stands out the most and for the KISS rule I’ll leave it at that. Best of luck, cheers.