1998 Volkswagen GTI Why the radiator high speed fan not kic

In the past my 98 gti vr6 high speed fan used to kick in whenever the temperature reached 198 degrees, but now it no longer kicks in.I replaced the radiator fan switch, the fan control module.I also change the engine oil, the spark plugs. Could it be possible for the thermostat or the cooling temperature sensor to prevent the high speed fan for kicking in?I checked the fan which is turning smoothly. Could the radiator be clogged?Please help, guys.