Look on the heater hoses for a ball and lever valve. This is the control for the heater core flow which allows a certain amount of flow depending on how hot you have the heat setting on. Make sure the cable is attached and that the lever moves without a lot of effort, you cannot move it much just wiggle it to see if it is free, and make sure that the clamp that holds the cable housing is clamped in place whcih is right next to the ball valve.
If this alll loooks good, there is a bable, the same cable that is attached to the heat ball valve, that is attached to the heat control ****. Make sure that the heat control is actually moving the ball valve by having someone turn the heat temperature up and down while watching the valve to see if it moves with the changes made at the dash panel. If it does then there might be some blockage in the coolant line or the preesure for the whole cooling system may be bad, if you have not checked that do to make sure. If it does not move the ball check valve see if the cable moves as that wil indicate that it is connected to the heat control on the panel end at least. Then the issue is with the ball valve as you may see visually or you should check ny removing cable at bvalve and trying to move it by hand. Sometimes they seize and will need replacement.