Renault Clio Overheating?

hi there. I have a 2003 1.4 16V Mk2 renault clio. the car has recently been overheating and the water in the radiator sump would boil, but the temperature reading on the dashboard would not rise and the fan would not kick in. I've had the fan replaced and the temperature sender, but the same problem keeps happening. the fan fail-safe will kick in when disconnected, so the fan seems to be OK, but why won't it kick in when i'm driving?