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If your blend doors are working correctly as you indicate it leaves two possibilities, one is low coolant and air is accumulating in the heater core as it is a high point. Usually if this is the cause it will get better if you are driving as the coolant will circulate better.
The other cause will be the heater core getting plugged up so that the capacity is greatly reduced. The coolant will still flow though but the core is essentially smaller. If you turn the blower off for a couple minutes and then turn it back on it should feel hot for at least a few seconds. You can try removing both heater core hoses and flushing the heater core in both directions. If this does not work you would need to replace the heater core which requires removing the dash.