There are lots of reasons for failure to thrive - things like reflux (true reflux, not just and unsettled baby), congenital deformations like a hole in the heart that needs surgery, tongue tie and poor latch, or PCOS and other hormonal problems leading to low milk supply, and so many more. (breastfeeding-babies.com)
If these things do not help then it's important to look at estrogen dominance issues often seen in PCOS and other hormonal problems including endometriosis and fibroids and determine if the uterine lining is responding excessively to estrogen. (betterbydrbrooke.com)
Other hormonal problems related to the ovaries like progesterone levels may require a bit more detective work. (breedingbusiness.com)