Nic - Em is 17 months and has been around 10 kg since before she turned 1. As they get more active it gets harder to get them to gain weight. She is not listless and still uses her nappies from what you have said so she is not failing to thrive, she is just little. Walking does take a lot of her energy still so just keep having food available to her and she will eat when she is hungry. If she was having trouble getting enough energy she would have no thighs and no bum cheeks. I am sure it is just a precaution, and they just want to make sure there is nothing that could be causing her to stay the same weight apart from her energy use and excellent metabolism.