2005 Grand Caravan A/C automatic temperature control

Now that the outside temperature is more summer-like, I have noticed that the air conditioning in my 2005 Dodge Grand Caravan appears to be working but is somewhat weak, but I tend to think that it is somehow
related to the automatic temperature control rather than the A/C itself. Here's the symptom: On a sunny day with the air temperature at 83 degrees, the interior is obviously much warmer, and if I set the automatic temperature control (ATC) to, say, 74 degrees on all three areas (driver, rear, and passenger), the vents on the the driver's side seem stuck on 'heat', blowing hot air, the vents on the passenger side are somewhat cooler, but the headliner vents in the rear compartment seem to have a nice cool air temperature, which leads me to believe that the problem is more with the ATC than the air conditioner itself.
I mentioned to my dealer a few months ago that I thought the A/C was acting kind of weak...they checked the A/C over and found it to be operating OK. Now that the outside air temps are warmer, maybe it is related to the ATC.
Just wondering if anyone knows of a Chrysler service bulletin or has experienced a similar problem. I plan to contact my dealer this coming week, but I'd like to point him in the right direction.
Thanks in advance for any response.
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Could be slightly low on charge.
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