Blows cold on defrost but with switched to the other vent controls won’t blow out??? Just did a coolant flush too not running hot!!

 

You might a vacuum leak.

 

Pop the hood and walk around to the passenger side and look by the firewall. You're going to see where the evaporator core and heater core intake/exhaust tubes stick out. There is a hose that passes through the firewall and enters the dash around there. It branches off a T fitting. It is possible that the hose or fitting is damaged or simply disconnected.