Why is your heater not working after you flushed the heater core?

If your heater is not working after flushing the heater core, it could be due to air trapped in the cooling system, which can prevent proper circulation of coolant. Additionally, there may be blockages or debris that weren't cleared during the flush, affecting flow. It's also possible that the thermostat is stuck or that the heater core itself has sustained damage. Checking for these issues can help identify the cause.