Stoves combined Fan switch/thermostat

Need a small question answered, I’ve recently replaced my fan oven thermostat, it’s a combined swith/therm unit. The initial problem I had was over temperature.
I marked all the wires prior to refitting the said new part and quietly confident refit all panels and pushed the oven back in place as the fan ran when switched on. It was only 15 minutes later that I found my well earned pizza still frozen and the oven still cold with fan running happily.
Q, where the thermostat metal outer runs through the body of the oven, does need to be insulated. I know it works as a bi-metal strip principle and could this be my problem.
Otherwise buggered new part and I’ll have to get another one. Thanks Figs