You could have an engine not getting to operating temperature. Check the temperature of the hoses to and from the heater core. Are they both cold? Both hot? Is only one hot? If they are both cold, then you could have a thermostat sticking open. If they are both hot, then instead, you may have an issue with temperature control in the dash (like a faulty temperature blend door motor...). If one is hot and one is cold, you could have a heater core restriction.