I noticed mine was rejecting a particular brand and when I tried another a few days later he was okay with it. He is still mainly on BM but takes 1 feed of FM on weekdays ;)
Hi... you can try to introduce solids to your baby to supplement. Try mixing BM with FM. Initially a quarter FM, 3/4 BM. If your child drink, you can increase FM gradually
Start by putting in small amounts of FM in your BM, then gradually adjust the amount till it becomes fully FM
Transition slowly. Do a mix of BM & FM, and lower amount of BM each time. If doesn’t work, switch brand.