Why is coolant not returning to the engine as the engine cools?

i would say if the coolant is getting hot in the engine but not flowing threw the radiator to cool the engine down the thermostat is stuck closed and not letting the fluid pass OR Ive also heard that if a air bubble gets caught in there it wont feel the hot fluid and open to flow the coolant threw the engine im pretty sure in the z31 the thermostat is behind the pulley that goes to the a/c pump you have to take that off and you will see were it goes in the engine i would change that out and see if that fixes it