I had one who sounds a little like what you are describing, and a couple of weeks ago she was looking pretty puny. I mixed up some vitamins and electrolytes and put them in a ricotta cheese tub with a hole cut out of the top (about half full).
I sat with them (Brooder II is 16 sf, so I can get in there). I picked her up and cradled her, and offered her the v&e mix. She settled down and began sipping down some mix. I just held her for a few minutes, close to the rest of the brood. I chatted with her, just held her.
That was late one evening. By the next morning she was perked up quite a bit, running around a bit more. I make sure that when I feed them salad, that there is an extra little bowl I can put in front of her if she chooses not to jump in there with the rest.
Lately, while she is still the shy and retiring one, she will elbow in there when the food or fresh water is put in, she now holds her own in the salad frenzy.
So, I think some extra TLC can help a shy one manage to thrive. She is still the smallest, and a bit slower to develop than the rest, but otherwise seems past the crisis point.