Does the temp gauge show that the engine is heating up? If not the the thermostat may be stuck open.

If the engine is getting warm (180) or so degrees then perhaps the heater core is stopped up not allowing the hot coolant to flow through it.

Does it have enough coolant in the radiator? If it is low it won't circulate to the heater very well.

My bet is a stopped up or plugged heater core.