Why won't the coolant in the engine block get warm after a new thermostat was installed?

Chances are you placed the thermostat in backwards or there isn't enough coolant in the system to circulate thru the engine. You may have a bad water pump. But first check your temp sending unit. If your temp gauge doesnt go up at all, thent hats the problem, its your temp sending sensor, replace it answer check radiator cap, u may need a new one have been known to buy a bad thermostat when it was new check water pump with engine running above idle, and at running temp. remove radiator cap and look in radiator if water is moving, your pump is good so is thermostat if no movement is thermostat or pump u might also chck to see if air is trapped in cooling system, a lot of new vehicles need system to be bled of air after doing a repair HINT if you have a coolant overflow tank, most vehicles need coolant to be added thru this tank and not thru radiator, this being done to stop overfilling of radiator and excess air from getting in system