I have a friend who had her twins exactly one year after I had mine and she's also breastfeeding But the smaller one is not gaining weight, and has even lost weight. (They are 6 weeks now)
She supplemented her for a week by expressing after every feed and giving it to her in a cup. Then she gained again, but since stopping now, she's lost weight. Now she's giving her a bottle of formula in the morning, 2 hours after she breastfed.
She asked me now if her breastmilk might not be good for her and I just cannot think that that could ever be the case, for any baby. If breastmilk from another mommy that's not a baby's own is better than other milk, why would it not be good for her own baby?
My one baby failed to thrive as well in the beginning and what I did was spend lots more time skin-to-skin (I believe they need physical touch just as much as food to thrive), and on advice from my IBCLC I fed her on the same breast twice before switching them, pressed my breast slightly while she sucked to help her get more hindmilk and if she wanted, let her feed on her sister's side after she fed as well.
Is there anything else you would suggest?