I would be waking her and getting her back to 2-3 hourly feeding during the day and then waking her at night - if she misses a feed get up and wake her up anyway. The other thing I wanted to say was that this may just be her and the way she gains weight. One problem I had with my second bub was that she didn't gain the mountains of weight that my first bub did and I couldn't help but compare the two of them. Once I accepted that she was never going to be a massively chubby baby and gain a heap of weight each week, it made it so much easier to see small weight gains without thinking anything was wrong with her. You're in a similar situation DD1 was a chubby little thing and gained weight well, but DD2 is a different baby and is also fully BF and BF babies don't always stack on the weight like FF bubs can. I still think you are doing a fabulous job though!