Temperature sending unit problem.

You could do some testing with a ohms meter. Check the senders base to neg' battery terminal. You're looking for near zero resistance. As stated above, a lot of thread tape can effect this path. Most new senders have a painted coat of red colored sealant on the threads. That should be all it needs.
Start the engine and let it warm up. When the 180 degree thermostat opens the sender contact post to neg' battery terminal should be somewhere near 34 ohms. If that checks good, hook up the sender wire. Gauge needle should climb to low end of normal range.
Have you considered thermostat stuck open?