if the gauge is not getting upto half I would suspect the thermostat valve is bad or completely missing..
Of one experience I can tell you with a cuore and also with a margalla and with a mitsu galant GDI too - the water pump (waterbody) impeller worked loose and off the shaft, there was no movement of the coolant when the engine started. and the engines would heat up for no given reason.
in one case I have seen a cuore waterpump impeller that was so badly designed that it was impossible for it to move any coolant at all. You also may want to check the water pump V belt, it may look perfect but maybe slipping because of glazing.