How could a parent not realize their baby was starving to death?

Article about a woman who wasn’t making enough breast milk, or not making any at all, and didn’t realize it. Her baby starved.

OK, new mothers-to-be talk to people. They read books. I did. I talked to everyone. They told me I’ll know if my milk comes in, and boy, did I.
I could feel it rushing when it was time to feed. My breasts became swollen and hurt if I missed a feeding.

My mother was visiting, hovering over me, watching me nurse, and she expressed concerns about whether the baby was getting enough because you couldn’t see how much she ate (she never nursed. Psychological issues with it, probably.)
She wouldn’t let up. “How do you know?? How do you know??” I got exasperated and pulled the baby off and shot a stream of milk in her face! LOL! She jumped back and said, “Well, I guess she’s eating!”

Even if it’s your first, and you don’t know to expect all that, how could you not see the problem? How could the doctor not catch it at the newborn’s first check up? How could all of support systems out there have missed it?