I had similar problems with my DD2, she had very small weight gains once she reached 4 months.
No one was really worried as she was alert, happy reaching all the milestones etc.. In the end our problem was that my iron levels were low & that affected my supply.
Even now DD2 is still smallish she was 8kg at 12months, so she is just a small gainer.

I hope you daughter has a nice big weight gain soon, i know how stressful it can be.