You may have an air-mix door that is not functioning properly. It isn't operating to allow air flow to change direction and go through the other vent locations.
1. A trap door can simply be jamed. 2. You could have a bad vacume hose/connection, some use a vacume to open and close trap doors. Particularly from the floor vent to the upper vents 3. Some use electrical componets, an actuator to open and close doors.
Answers & Comments
You may have an air-mix door that is not functioning properly. It isn't operating to allow air flow to change direction and go through the other vent locations.
1. A trap door can simply be jamed.
2. You could have a bad vacume hose/connection, some use a vacume to open and close trap doors. Particularly from the floor vent to the upper vents
3. Some use electrical componets, an actuator to open and close doors.
This is very likely an easy repair.