Snug, based on what you say in your last post, it sounds that she was just not feeding sufficiently often in those early days - someone should have pointed this out to you. 6 hours is too long to go between feeds at any time for most babies.
Not sure how long that persisted - maybe until day 4-5 when she was seen to have lost 10 per cent?
Whatever...can you start feeding her more often and giving at least both sides each time? She is still not feeding anything like often enough to address slow weight gain - at a rough guess 8 times in 24 hours? That's right at the lower end of frequency for a young bf baby, and if you could bump that right up and increase the switch nursing, this would have a good effect on her weight, withing just a few days.
The reason behind most instances of worryingly slow weight gain is usually the bog-standard simple one - the baby is not getting enough chances to feed.
Some babies are laid-back and chilled, and don't 'ask' for feeds as often as they might - they take in sufficient to tick along, and avoid any serious under-feeding, but they don't get sufficient to really grow as they should. Could be babysnug is in that category....it's usually easy enough to turn round, though, once you really start feeding more. You may need to concentrate on this for the next few days so she turns a corner (help with your other dc is going to be important, I would guess).
Pumping, in these circumstances, is an added thing to do - worth doing, but it's not as important as getting more milk down direct :)