2009 Ford Explorer Sport Trac XLT air flow stuck on front vents

We just purchased a 2009 Ford Explorer Sport Trac XLT pickup for our teen son yesterday. On our way home, We had the AC going and it works perfectly. But as it got humid outside, the windshield fogged up and didn’t have any wiper fluid in it. But when I tured the air flow selector knob to the defrost setting, the ac kept coming out of the front air vents. barely anything out of the defrost vents. So I changed it to straight floor vents, and nothing came out of the floor vents…still only through the front vents. Any ideas on the issue? Want my teenager to be able to defrost his windshield when needed driving and be safe.