What might cause a delay in milk production?
First off, keep in mind that although some people do seem to take a little longer than others, you would expect to see everyone's milk transition by around 6 days.
Milk delays coming in for 2 main reasons:
1) A hormonal one e.g. a c-section prevented the natural kick-starting of the process with hormones that are around during labour or another hormonal cause such as some of the placenta being retained.
2) The second - most common reason - the breasts not being stimulated post-labour with either baby sucking or pumping as a substitute. The baby's sucking actually comes down to another hormonal reason as that's about the prolactin hormone being stimulated. Perhaps if a baby was having sucking problems, had low energy due to low blood sugar, was excessively sleepy, was supplemented immediately so had less demand for the breast - these would all delay milk coming in. A poor latch would also mean breasts don't get stimulated properly.
A continuously poor latch may mean milk never really comes in effectively. This combined with feeding that wasn't frequent enough would result in a long-term problem. Latching isn't just about what a breastfeed looks like from the outside. An LC needs to also look inside a baby's mouth to look at position of tongue and shape of baby's palate which can also affect latch.
There are some conditions such as PCOS which can also contribute to hormonal problems and lead to milk supply issues. Or perhaps another hormonal cause like a mother starting the combined contraceptive pill.
Or another medication - like an anti-histimine or certain decongestants.
Women with low thyroid (a problem that can develop during pregnancy) or a mother with anemia may also have supply problems.
Obviously a mother who has had breast surgery such as reduction or some other kind of surgery may also have problems.
In some very very very rare cases (not even 0.5 of 1% of the population) - a mother may have insufficient breast tissue. Their breasts may not change during pregnancy and there's no real leaking or engorgement, perhaps in adolescence one breast didn't really develop at all. Breasts may be tubular in appearance or extremely underdeveloped. I can't stress enough how unusual this is but I am mentioning it. This is not to say small breasts lead to problems - not at all - but people with very small breasts will see a change in pregnancy for sure.
You can read more about insufficient breast tissue and some solutions here:http://kellymom.com/bf/got-milk/supply-worries/insufficient-glandular-tissue/
However any breastfeeding consultant would obviously see your breasts and ask you questions about your breasts in pregnancy - this one is fairly easy to identify as it's fairly distinctive.
(Information originally written by "Samuel's mum" in the thread Making breastfeeding work second time round - 'Milk Never Came'