the box that holds the components for the inside a/c parts have mixing doors in it to allow heat or cold or a mix for defrost etc..
these doors are moved by either vacuum pots or motors.
normally vacuum but newer cars might be different.
sometimes those pots go bad. you can see the actuator arms from the outside of the box but you have to lay down upside down and look up into see most of them.
a mirror on a stick can help too.
by moving the knob/lever or whatever yours has from heat to cold, or just starting the car and shutting it off you can see if they move like they are supposed to.
based on your symptoms, one of the doors doesn't move.
unless you have zone heating and cooling.