The thermostat is not opening

If you know for certain the thermostat is not opening then you may have air in the system. You will need to bleed it out. I suggest renting or buying a vacuum bleeder. This is a tool that puts the cooling system in a vacuum and then you suck the coolant in. This eliminated the possibility of air. If you know it has no air then make sure the fan is coming on and the radiator and condenser isn't blocked or dirty. Alex