1998 Buick Park Avenue Only partial cooling

Trying to find out why I only get cool air from the two dash vents closest to the driver and warm/hot air from the two vents closest to the passenger & rear vent at end of console. The passenger climate control (slide switch on dash near passenger) lights up, but seems to have no control). The hotter the outside air, the hotter the inside air on the passenger side.
This is actually a Buick Ultra, if that makes a difference
Thanks, in advance, for your help