HVAC Fan works but no air

[buzzboy]

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.

[bazar01]

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

[buzzboy]

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