Buy her out, sleep, then go find her again
I found the 3 cowmilks, bought all the stamina supply of the cowgirl slums, even slept.. but it still shows 5.
I also reseted (used the bell) and invited 6 cowgirls again, bought all the stamina supply, it still says I have only 5 cowgirls have joined.
One of them doesn't immediately go with you. Sleep, then look around the slums again!