I was a cow. Moo. My son nursed an average of 20oz from a few weeks to 9mo PER FEEDING (outlier, most don't). I also wet nursed from time to time for a friend who needed surgery.
But I couldn't pump.
I cried. The lactation consultants cried. We tried EVERY machine over several months... because I would have made a KILLER donor I had so much supply, and my supply was so versatile, but it would take an hour to get an ounce, and the tap would turn off. Not a single drop. Didn't matter prenursing, postnursing, expressing, hand pump, fancy schmancy hospital machines, beer... just. couldn't. do. it. But put a mouth to nipple, and whoosh. <rolls eyes>
Some women just can't pump. Some can for 1 or more births, but not for another.
But DO try a variety of pumps before giving up.