fan over run not working

Hi. I know there have been posts on this topic here before, but I can't see my specific issue addressed. I'm replacing a bathroom extractor fan. The old one [EnviroVent silent 100] worked fine including the timer overrun, but I'm redecorating so I thought I'd replace with a better looking and...

We have installed a new bathroom fan with an isolator switch. The fan comes on when the lights are on and off when switched off. Issue being the fan wont continue to run after the lights are off. It also has a humidity sensor built in, we have tried turning the timer/humidity screws both ways...