the thermostat need "boiling water" in an outside test. in a closed system water will boil at a lower temp. Thermostats usually have a temp on the outside of them when they operate, usually about 87c for most VAG cars afaik. fans only kick in when temp rises above the normal temps usually about 95c or so. maybe worth replacing the temp sensors as well, just in case one of them is failing.
you can get air blockages in the coolant system, you need to allow the temp to get to normal 90c then start squeezing the top and bottom rad pipes. you will feel the water within the pipes then it will be ok. if the thermostat is now working then water pressure should release any air blockages, just keep an eye on the water level in the header tank.