'95 suburban AC in rear seats

My '95 Suburban has very good A/C for the front seats. However, when you turn the A/C portion on for the rear seats, the air comes out warm.
I checked the owner's manual to make sure that the truck was supposed to create cold air in the back, and I do have the model that does both A/C and Heat in the rear of the truck.
Any ideas on what the cause of this might be?
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