i had this problem for about 2 days aswell. I would have s rank and be getting ss everytime after and it will not save. BUT IVE FINALLY FOUND A SOLUTION! the reason it wont save on top off the beatmap is because you must have the top score for that beatmap. I had my s rank a higher score than the ss rank and then I added a few difficulties and instantly I got it.
hope this helps ^_^