When she's feeding, are you seeing some good spells of sucking/swallowing? Just because you aren't necessarily feeling the letdown doesn't mean it's not happening. The suck suck swallow pattern is a better indicator of whether milk is being transferred.
The one thing I'd wondered about (yes, I know I see it everywhere LOL!) is whether she might have a tongue tie. If, as you say, she seems to be feeding for ages, then dropping off/falling asleep and then waking soon after wanting more food, it could be that she's not able to get at the milk very well. Also the fact you've said your nipples were really sore (and also sore with DS) makes me wonder. How are they feeling now? It might be worth asking the midwife/breastfeeding advisor to have a feel and see what they say. If they just look and say they can't see anything, get a second opinion. You can't necessarily spot a tongue tie by just looking.