2 easy checks
 

1 is the bleeder thing I mentioned above, another is with the engine cold and the radiator full cap off start your car, if water shoots out of the radiator you have a blown head gasket. hopefully that is not your problem but air prevents the thermostat from opening and aslo could prevent the heat sensor from heating up depending on it's location, good luck I know what it's like to be stuck with a problem and no answer seems to help. I wish you my best.
Jamo