No heat, vents blow cold air

I have flushed heater core, checked actuator, checked coolant, bled air out of system, checked thermostat, checked all hoses. I had just warm air after flushing and then drove car for a mile and heat quit blowing only cold air from vents. The temperature stays at a constant 210 when sitting running or driving, the blower works, no clicking noises. The vents in the center seem cooler and the is a little warm air that comes out on the floor when driving but no other heat. What else can I check?