It is probably because you don't enter the right character.
It is ' (vertical) and not ‘ (inclined)
and it is " (vertical) and not “ (inclined)
But I realise that with iOS 11, it could be difficult to find this character depending of kind of devis (iPhone or iPad).
On iPad with an english keyboard, you don't have directly the ' and " but the ‘ and the “.
You have to do a long press on ‘ and “ to have more choices and choose ' or ".
I think that I have to add the possibility to enter ‘ or “ (both inclined) instead of ' and " (both vertical).