For me it was after we'd been on a couple of dates and banged a few times, she calls MC up and asks about how he views the relationship, you either become bf/gf or she sort of ghosts you for the majority of the game - a few appearances, but also missing, like for the gym photoshoot.
I didn't go after Andrea and let her keep studying basically, didn't want to rock that boat. Tatiana is one that I had a lot of interest in, but apparently my character got to a 'no shot' dialogue moment. Not sure if I screwed something up, or missed something.