When the engine is hot check that both heater hoses going into the firewall are hot. If they both are hot then check the door that directs heat or cold air behind dash board and make sure it is moving and changing air temperature.
Two things that can cause this
Restriction in cooling system specifically the heater core or the blend door inside the air conditioning box behind the dash board is stuck on cold and is not redirecting hot air out of dash vents.
Thanks for the question