Why would my car's heater not make the air warm if i already changed the thermostat ?

i changed the thermostat and i am still not getting any warm air from my heater what else could it be?|||Blocked matrix.Try removing the pipes and flushing with a hose pipe.|||Plugged heater core?|||i truly can help butttttttttttttttt year make model engine trans miles|||other possibilities:





Broken valve or heater control cable.


Air lock- air in the heater system ( not that likely)


plugged up heater core- most likely issue|||Engine coolant not moving--A quick test feel the top Rad. Hose when Eng reaches mid range.Hot that is good-Not then air-lock,---the fix, disconnect top rad. hose at Rad.start eng. you should get Eng.Coolant;