For me, and I've heard this for others as well, the more traumatic micro births can cause a delay in milk coming in. Even for a FT kid it can take up to a week though. I was pumping religiously ever 3hrs with my 29wkr and my milk came in on day 2-3. Then with my 25wkr where I was knocked out via general for my c/s and had a post-partum infection, my milk didn't come in until day 5-7 but even then there was never an overnight thing. It was very gradual.