Some things to check. Are the heater hoses warm? if not check for blockage at water pump, heater valve ( valve inline on the heater hose that allows flow to the core) or blocked core itself. If so then check the air mix door inside the cab under the passengers side to make sure it is allowing air flow through the core and blocking the ac and outside air. Low coolant, pluged heater core, feel the hoses to see if they are both warm, door closing outside air not closed. Cable or vacumn pot not closing it.