Mould despite fan, windows, paint etc

[Youtani]

So fed up off getting mould on my bathroom ceiling. Have painted with anti mould paint, have installed a powerful extractor fan which stays on for an hour after showers and have the window open. What can I do?

More insulation in the space above?

As first poster said, more insulation. My bathroom ceiling slopes. When I did the bathroom out, I had the ceiling taken down, had kinspan put in and then recorded. The mould never returned.