I think you can look at a few positives...she is gaining (it may be slowly but she is gaining), she is happy and healthy, she is developing well and she is eating so getting nutrients.
DS4 is/was FTT, he is currently 7.3kg at 18mths (he has not been on the charts for a long time). We have had every test under the sun to see if there is a problem...tested for CF, metobolic disorders etc.
We are under a dietitian at the hospital and have been for some time.
They got really concerned when he started losing weight...but at the moment our pead is reasonable happy because he is happy, healthy and doing well developmentally...just tiny
He seems to have a fat intolerance and that may have something to do with it.

I really understand your concern and but she may just be a little girl with a really good metabolism.