Help....No Heat

I’m not getting any heat blowing when I turn the heat control potentiometer to hot. When I turn the knob to the cold position the A/C works fine.
I removed the glove box to examine the operation of the Air Mixing Damper Control Motor. When I turn the knob from cold to hot, the Control Motor rotates from the Max Cool position to the Max Hot position just like it is supposed to. I turned the temperature select knob back and forth and the Control Motor reacts correspondingly.
The heater hose connections at the damper control unit are very hot to the touch when the engine is running so the hot water must be flowing OK
So if the Air Mixing Damper Control Motor operates correctly from Max Cool to Max Hot and the hot water is flowing in and out, then what else could be causing no heat to blow?

heater shut off valve?

I was not aware of heater shut off valve. Where is it located?

Are you refering to the flow rate control valve in the heater unit?

I'm going with what we used to call your flapper is stuck, every car has one it's just a matter of getting to it, you were in the rite spot when you took off your glove box, there are tiny vacuum lines back there, track down the one that controls the pneumatic function for opening the air transfer box. bodabing! you done.