Have an 04 Hyundai Sonata 4cyl that the heater blows cold air. Coolant was very bad and radiator was rusted when I bought it. Flushed heater core 3 times backwards and forward with air and water. Was initially blocked but flows clear now; no blockage. Replaced the radiator. Replaced therostat. Replaced water pump. Flushed system and replaced coolant. Checked actuator and blend door is all working fine. Checked all my fuses. No leakes anywhere. What am I missing?

How about purging the air from the cooling system?

Let vehicle run and reach operating temp then feel the two hoses coming from heater core if one is hot and other one still cold or lukewarm heater core is still clogged most likely needs to be replaced since you already flushed it three times.

put the car on ramps with engine running, radiator cap off heat on full blast keep filling to all air bubbles are gone happen to me the engine doesn't fill all the way because engine sits little higher then the radiator with car on ramps the coolant will go threw heater core, make sure engine is hot so thermostat is opens before you start filling so all the air bubble come out.