Ok, I got it. Finally. It's a matter of principle. So here is the trick...It was just and option during a normal convo, I believe she was in the interior studying.
If Rachel's mood is 2 or higher she won't spank Chloe. When I finally got it I had cheated and had Rachel's mood at 3 red.
Do you have Chloe doing handjob in hottub?
Are you watching all the way through? or leaving at some point to avoid being discovered?
You must have spanked her 3 times or more. I had that.
You must have unlocked hot tub hj. Had that.
But what the WT doesn't say is that you need to have unlocked ALL stars in the hot tub with Chloe. Hot tub HJ comes in 2 versions, and both of them give one star. You need to have both of them. The "final" version is her bending over, which happens if her mood is 2+. However you need to unlock also the version when her mood is less than 2. Once I had that, I got the conversation option, just like you wrote.