Why are the driver's side vents not getting heat and passengers is?

When the temperature control knob is turned to 100% heat, the air flowing out of the driver’s side vents are warm/cold when the passenger’s side vents are blowing hot air. The apparatus that controls the mixture of cold and hot air works correctly, I will mention that I have noticed it to skip out of place. But even when it is working correctly I still have warm/cold air blowing out of the driver’s side. (all vents on driver’s side)