My 2012 Jeep Liberty doesnt blow warm air

I have taken it to 2 different mechanics. The first one changed the thermostat. The next fix was replacing the head gasket and radiator. That still didnt fix the problem. I have flushed the system about 5 times. After each flush, the heater starts working immediatly, and works for about 2-3 weeks and then blows cold again. I've using a power washer to flush the system, and this last attempt, used CLR to flush out any residue. I've tried elevating the engine by driving up a steep hill and reving the engine totry to get any air bubbles out of the system. I am at a loss and am not sure what else to try- short of trading the car in or bundle up.