I have to say that i too havea bub that struggles with weight, she was fine untill the 6mnth mark and then she all of a sudden was losing weight - now she is 18 months and still only 9kgs!
I ended up having monthly weigh ins for about 6 months and now we are down to 3monthly. I actually have one this coming week.
On the charts she has always been well below everything for weight but her height has been around average and she has always been happy and healthy and ahead in mile stones. I b/f her untill about 13 months, mainly because it was the main thing i could get into her - she wouls pick at solids one day and then not want anything to do with them for a few etc - very frustrating - as you would know! I think she didnt really take to solids untill 11mths and even now she will havea few days where she isnt keen on anything and then she makes up for it later - i just offer her snacks all day long!

I was able to put tori onto a formula, just at night, when i stopped b/f and she seemed to keep a stable weight then, this one was called pediasure and is a complete nutritional, vit/mineral supplement, vanilla flavour - you get it at the chemist warehouse - it is even used for peg feeding kids. I'm not sure if you could use it on under 12mth olds though but i found it really great - can be used on up to 10yos! I get it when tori isnt well, just so i know that even if she isnt eating she is still getting some form of vitamins etc. Sounds like i am doing an add for the stuff but im not, i just had success with it.

Just know that you are definately not alone on this one! There is nothing worse either than how everyone seems to comment on how small your bub is etc.
Good luck with your weigh in this week - let us know how you go, remember that if she seems happy then she must be doing alright!