HVAC Fan works but no air
 

When I start my car up the HVAC works great. Brand new heater relay and the fan blows strong. Air will blow out of any vent that I tell it to. As the car runs though, air stops coming out of the vents. The fan is still turning and I hear air being moved around but it seems like the flaps all close themselves. I have no idea even where to start on this one. I would almost be okay keeping it so the defrost vents were always open as they're the ones I care about anyway.

Probably low on refrigerant causing the evaporator coil to ice up blocking air flow.
Recharge then test for leaks.

I've never had this problem with the AC. Only with the heat.