If I remember correctly, you need to bring her a drink and then visit her later in the shower. This will give you an additional scene with the stats increasing.
Thanks for your answer. I tried giving her all kind of drinks but nothing happened. I think you are referring to the step when her sluttiness is level 4 already as per the walkthrough. She is currently at 2 for me and I can’t increase it any further. The walkthrough suggests that I just need to watch her shower but nothing changes?