Secondary causes are far more common than primary ones. One study found that 15% of healthy first-time mothers had low milk supply 2–3 weeks after birth, with secondary causes accounting for at least two-thirds of those cases.Breastfeeding management issues that can interfere with regular milk drainage from the breast include poor latch, unnecessary use of supplemental formula, timed or scheduled feedings (as opposed to on-demand feedings), and overuse of pacifiers. Medical conditions in the infant that result in weak or unco-ordinated sucking can cause low milk supply by inhibiting the transfer of milk to the baby. These conditions include tongue-tie, congenital heart defects,  prematurity, and Down syndrome.Primary causes of low milk supply include:

insufficient glandular tissue, also known as primary lactation failure. Breasts with insufficient glandular tissue often have a tubular shape, with a large space between the breasts, and often do not grow during pregnancy. retained placenta
Sheehans syndrome
prior breast surgery (especially breast reduction)
prior nipple piercing
polycystic ovary syndrome (PCOS)
hypothyroidism
hypoprolactinemia
theca lutein cysts
hypertension
pregnancySmoking more than 15 cigarettes per day is associated with reduced milk production. Many medications are known to significantly suppress milk production, including pseudoephedrine, diuretics, and contraceptives that contain estrogen. It is suspected that some herbs, particularly sage and peppermint, suppress milk production. Mechanism
The physiological mechanisms that regulate breast milk supply are poorly understood. High levels of prolactin are necessary for lactation, however there is no direct correlation between baseline levels of prolactin and quantity of milk production.
The infant should be consistently gaining weight and growing. In newborns, a loss of more than 5 to 7 percent of birth weight warrants investigation. The use of IV fluids in labour tends to artificially increase the birth weight of the baby, and subsequently inflate the babys weight loss. Newborns should regain their birth weight by two weeks of age, and gain at least 150 g per week.If an infant is not showing these signs of growing well, other possible causes include:

Poor milk transfer due to tongue-tie, or cleft palate. Medical conditions in the infant, such as malabsorption of nutrients. Definition
A mother is considered to have low milk supply if she produces less breast milk than her infant requires. The term is used only after a mothers milk "comes in", which usually occurs around 30–40 hours after delivery of a full-term infant. Low milk supply is distinct from the scenario in which the mothers milk comes in later than normal but is thereafter produced in sufficient quantities; this is known as delayed onset of lactation (delayed lactogenesis II). Prevention
The first week after birth is a critical window for establishing milk supply. The Ten Steps of the Baby Friendly Hospital Initiative describe some maternity hospital practices that promote the development of a good milk supply, such as rooming in (allowing mothers and infants to remain together 24 hours a day) and having trained staff available to help mothers with breastfeeding. Supplemental formula or donor milk should be given only when medically necessary.