ac is warm on driver side, but cool on passenger side on 2003 Buick LeSabre

AC is not very cool on driver side, but cool on passenger side. Dealer added freon previously and it improved, but is happening again. Why would this happen?

Asked by for the 2003 Buick LeSabre
If your car has driver/passenger temp settings then that's the problem. A "flap" that controls the amount of cool/warm air going to one side of the dash is not working properly.
1 more answer
Had same problem, 2003 Buick Century, tried all the various efforts posted.
Found that the Temperature controllers (potentiometers) can be easily broken when removing the knobs. Place sliders at mid range before attempting removal. The bodies are not soldered to the PCB. Some means of appling pressure to the body of the potentiometer helps. The actuators turn 90 degrees (1/4 turn ) the carpeted cardboard under the dash must be removed to view. They are located approximately behind the inner vents a flash light is handy for viewing. There is a slotted shaft about 5/16 diameter towards the outside, watch for movement when the controls are adjusted from max cold to hot. My scanner showed no codes.
Measured temperatures where 18 degrees C passenger side and 23 degrees C driver side. My solutions was low freon after adding 8.7 oz temperatures are 7.5 degrees and 8 degrees with outside air at 83 degrees F. Good luck in your repair. gg