For Bell, you'll need to complete the pyramid quest. For Alissa and Cassie, try making Cassie change clothes multiple times. She can't be pregnant for this
I meant for Bell I already have done the pyramid quest, and I've given her the life water.
Isn't that enough to be able to invite her to the milking machine?
If not, you can go to the next followup quest after inviting all of the cowgirls.
Is the cowgirl in the devil's drink invitable?
Yes. (That's the one that sells a bunch of overpriced drinks, right? They have limited supply, so buy it out.)
