I have a two story home, using one single furnace unit which is located upstairs. Its a dual zone setup, with one upstairs and one downstairs. Recently it stopped switching over to heating downstairs, and only heats upstairs no matter what zone is selected on the thermostats. Im guessing somethings jammed, but have no clue where to look, or what to look for really.
There are coiled ducts throughout the attic for the upstairs, not sure whats inbetween floors for the downstairs.

Should i crawl into the attic and look for something in specific? Ive looked in the fiber glass box before ontop of the furnace unit, that has two ducts running out of it, and dont see any type of switching setup, just goes straight into both.

Any thoughts which can save me some time and steer me toward the right path?