I've had 2 women with milk that was very slow to build.  One "felt her
milk come in" as she drove home from taking her in-laws to the airport
on something like Day 9.  She nursed uneventfully thereafter.  With
her second baby, she needed to supplement until something like Day 6.
Faster than the first time, but still notably slow.  No reason found.

A second one started using an SNS, and "had to stop" on something like
Day 17 (I really have to go back and look that up!) "because there was
milk all over the place."  Midwife said placenta was definitely
intact.  This mom, too, nursed uneventfully from then on.  So yes, on
occasion milk can be *very slow* coming in, and everything can end up
fine.

It seems that substantial blood loss perinatally  (not full-blown
Sheehan's) can also put a temporary dent in supply;  I've seen a few
of those recently, and most went on to nurse without incident, though
it took weeks for their supplies to match their babies' needs.  One, a
mother of twins, didn't generate a full supply for them.  Two of the
others ended up with unusually hefty babies.  I think it took around 6
- 8 weeks for full supplies.

Diane Wiessinger, MS, IBCLC, LLLL  Ithaca, NY