Insufficient coolant , trapped air in the cooling system might not have enough coolant to full fill the heater core.
The hot coolant flows from the engine to the heater core through the "inlet" hose. After circulating through the core, the coolant returns to the water pump through the "outlet" or "return" hose. Both the inlet and outlet return hoses should feel hot when the engine is at normal operating temperature and the heater is on.
1. If the both heater core hoses were cooled, then this vehicle might have an insufficient coolant in the cooling system.
2. If the heater core inlet hose is hot and the outlet hose is cool, the heater core might be clogged.
Note: check the radiator coolant level and top it off as needed.
Some cooling systems have air bleed valves built into the system to aid in the removal of air pockets. Advice: For many cooling systems, the method of removing air pockets is to start the engine and allow it to run with the radiator cap off or loose until all the air escapes from the neck of the radiator. For some systems this method is not sufficient, and the manufacturer has installed air bleed valves usually near or on the thermostat housing.