Does she eat solids okay? At 15 months it's not unusual for bottle refusal. Why not try a straw cup or plain cup?

The underweight thing adds extra stress to you I went through a period of worry with my little miss too. I really ramped up the fatty good foods like avocado, cheese, ricotta etc. I am also guilty of making omelette with extra milk powder (not formula) and milkshakes with the same.