Hi. Trigger for Stella's growth is:
stats_skill_learned[1] == 1 and stella_strength_level == 3 and day.current > 71 and "town_event_1_completed" in topics_gina
Meaning you need the "Ginspy Roll" learned from Jade (dialogue at dojo), you need to have seen the Mike x Stella scene (dialogue with Mike or happens a week after delivering the potion automatically) and you need to have seen the Jade x Aline training events ( event-chain needs both finding an exploration event and doing a training with Aline on Gina - Strength_Level 4)
Unless the initial event hasn't started? You need either the potion set or the used set (aka. Transfer-Potion) in your inventory and visit her at the brothel.