Usually this is a problem with one of the "actuators" (little arms or servos that are controlled either by vacuum lines or electrically or by cables, depending). These actuators open and close "blending doors" which direct air or heat or AC to wherever you select.
So the question with your car becomes one of figuring out a) which door isn't opening and b) why it isn't opening.
Most blending doors are attached to the heater box under the dash. Often you can get under there and see various ducts or hoses going to the heater box---ducts for the defrost, and probably not much more than a trap door for the floor heat-- a flap essentially.
If you get under there and work your controls from the dash, or have someone do this, you might be able to see the problem.
If you can't, you're gonna need to either buy a workshop manual that shows you a diagram of the heater/ac system, or take it in and have it looked at.
Seems like you should be under warranty still?