The runny poop and pooping outside the box could definitely be food issues, and that could be contributing to her weight loss. Can you feed her separately from the others? If so, try starting her on a one-ingerdient-at-a-time diet, like either chicken or fish for a few days, see of her poop firms up. Then you can begin adding back other things, and see how she does with it. As a human with some food allergies and some intolerances, I know when I am battling a new one, I don't really trust food or want to eat much until I figure out what is safe again.