Envoy Air

I have automatic air on 2003 envoy. Ait works fine but it does not come out of the 4 vents on the dash. It comes out fine through the defrost vents and floor vents. What can be the problem?

Thank in advance

Welcome to AF, could be a bad diverter door control, or in the auto control head, a scan of the bcm with a bi-directional scanner can command the mode door positions to confirm the problem.