Defrost wont blow heat

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My floor and body vents blow hot but when switched to defrost no heat os present. I do have air flow but no heat. I'm thinking this is an actuator/door issue. This is a new issue after heater core replacement. Anyone have any input? FYI it's a 98 k2500 5.7 ecsb

 

[Courchained]

Update. Removed lower hvac cover and found heater core not securred to housing causing poor heat flow.