two story dual zone on one unit, not switching to other zone

[cezium]

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?

[firecontrol]

You've eliminated the easy part, the furnace must be working because the upstairs heats. Other than suggesting to double check that your thermostats are set correctly there really isn't anything else to suggest other than calling your service company. There are easily dozens of ways the system could be installed to zone the home and many dozens of different components that could be the problem.

[cezium]

You've eliminated the easy part, the furnace must be working because the upstairs heats. Other than suggesting to double check that your thermostats are set correctly there really isn't anything else to suggest other than calling your service company. There are easily dozens of ways the system could be installed to zone the home and many dozens of different components that could be the problem.

thermostats are fine, or look fine, i never call service companies lol. Ive managed to figure anything else that came up in the past, im sure ill figure this out too. Just was hoping to save myself some crawling in the attic if unecessary.