2000 Buick Park Avenue suspected blend door problem

i just reviewed a question posted on the site titled 1997 Buick Park Avenue blend door problem. The issue I have is exactly the same whereas the the AC is cold on the passenger side but lukewarm on the driver side. Removing the actuator and moving the door by hand makes no difference. The response to this question was either the system is low on refrigerant or the compressor is weak. Could you explain to me how this is possible? Thanks for your time.

R.Vare