your heater core may be stopped up, causing the coolant not to circulate. easy test. let the car warm up. touch the upper and lower heater core hoses (located on the fire wall, towards the middle yet of on the passenger side of the fire wall. if one is hot and one is not, your heater core is restricted/clogged. easy fix, you can bypass the heater core and just connect the two heater hoses together. professional fix, replace heater core. if both hoses are hot, that is not your problem.