I'm not sure if this will solve the problem, but I noticed a bug in the game with another girl where, despite the lubrication appearing to be enabled, the game doesn't actually recognize it. Try turning it off and on again directly before the act.
I must admit, I had a good laugh just now when we talk about realism in hentai games. But in the end, if this feature is what developers intended and people enjoy it, who am I to judge?... I was just a little curious if people really do enjoy this kind of gameplay