at what temp did it open? could just be the switch opening late but if it also doesnt with the new one in place then have you checked the 12V supply to the fan?
what temperature does the speedo show (fan only comes on over 100)? if it show more than that and does not come on it could be that your thermostat is not opening, water pump if not working (belt on wrong way around, impeller separated from shaft) or rad itself is blocked, hence not a lot of water coming through and it does not get over the set threshold there.
would go and first check 12v supply (measure at the plug, check fuse if nothing there), next check if your water pump is working, you should see a constant strem of water coming out of the small hose in the expansion tank. Check which side the belt drives the pump, if belt is the wrong way around it would run backwards.... (but it's possible to put on the wrong way around and still tighten it sufficiently so it SEEMS ok)
