If it's supply and demand, why doesn't it always work to have baby or pump on the breast more often?
I think on this front either the baby or the pump could be not effective so the 'demand' might not be happening even though the baby or pump is there all the time. Examples of this would be poor attachment, tongue tie, sleepy baby, pumps just being pumps and not being 100% effective. If you can't get a good attachment and a pump isn't cutting the mustard you are kind of screwed cause there is no other way of generating a good 'demand'.

I remember a woman at my mothers group who during our 2 hour session fed her DS 4 or 5 times. It turned out to be a tongue tie which was fixed eventually (like when he was 4 months old or something) but up until then he just couldn't attach properly, so would only get a small amount, then get hungry 30 minutes later. God knows what made her keep going until it god fixed, the woman must had had the patience of a saint and not like sleep! Anyway she was never getting to the totally empty breast stage which is very crucial in creating the 'demand'. So even when it did get fixed she had already missed a lot of the early establishing supply weeks and I guess you would say the feeding to boost supply was a lot less effective once the body had already had 4 months of getting the 'no you don't need to make any more milk' message.