OK, let's knock the appearance of breastmilk thing on the head once and for all :)
Different mothers do not produce milk that differs significantly in calorie/cream content. This has been shown again and again in the research.
However, if expressing is not removing milk truly effectively, then the cream content will be lower. This is because the cream content of the milk is highest when the breast is emptiest. So, if expressing is not really getting the milk out well, the creamier parts ('hindmilk') will still be in the breast.
The same goes for babies - if they are not removing milk as effectively as they might otherwise do, then the breast does not 'release' the creamier parts, and in the longer term, production dwindles because milk production depends on frequent effective milk removal - this does not mean the milk has to drain to the last drop, BTW. It is normal for milk to remain in the breast and in fact the breast never truly empties.
It is not the case that some mothers produce 'thin' milk and others don't. Their milk contents will be more or less the same as anyone else's.
Usually, the main thing that drives a baby's growth upwards is volume of milk - we don't have to worry about 'getting to the hindmilk', because if the baby is really feeding often and sucking effectively, this will happen anyway.
shelfy, what happened with your baby is so sad and you really struggled :( The heart of your story lies (probably) at the beginning, when milk production needs to get off to a good start with lots and lots of effective feeding, and I wonder if that happened with you. If not you may have been playing catch up for some time - and while some women and babies can cope with a difficult start and make up for it, some find it harder.