Aiyah, at 3+ months babies sometimes just get more efficient with feeding, they can extract the milk faster so they don't need the full amount, which is actually a good sign! But since your little one is borderline underweight, I'd really suggest checking with your pediatrician to rule out anything else and make sure she's gaining properly, because that's the main thing to monitor rather than finishing every drop. A few of us went through this and our doctors confirmed our babies were fine, but it's definitely worth that peace of mind given her weight situation. In the meantime, don't stress about the leftover milk, it just means her appetite is settling into what her body actually needs, though your doc can advise if you should be doing anything different with feeding.