Bathroom mold despite working in line strong extractor fan

coffeehound

Something else to bear in mind with extractor fans is they can only work well when there is a free supply of air into the room.  You might try slightly opening a window outside the bathrrom (landing / bedroom?), and then prop the bathroom door open just a bit to let the fresh air flow in without too much humidity escaping

Chickereeeee

If the mold is on the ceiling, it would indicate that is the coldest surface. Are you sure it is well insulated? Also ri0un-on of fan should be at least 15 minutes.