For the fan kit to come on, the entire engine needs to be hot enough (coolant and oil). When you shut it off, everything will equalize. If the oil is cooler, it helps bring the water temperature down. The main job of the kit is to keep coolant from beginning to boil in the heads, causing air pockets in the system.
If the kit works when you jump the wires on the sensor, make sure you have your sensor mounted correctly. The bottom of the sensor should be against the aluminum thermostat housing, with an aluminum angle sandwiched between if you don't have a timer model.
As a test, after you've driven the car for a while and are ready to stop, shut the a/c off and let the car idle (hood closed). When you hear the fan come on, shut the engine off right away. The factory fan comes on at 207 and shuts off at 200. Our kit comes on at 200 (at the sensor) and goes off at 180. The fan should come on within about a minute if the engine is hot enough. It takes a few moments for the heat from the exhaust manifolds to soak into the cylinder heads.
If you need further assistance, let us know.
Regards,
Sean