I have a lavender orpington (named Guinevere) chick that I bought from a breeder, along with 3 other pullets and a blue Orpington rooster, named Lancelot. Guin is still young, can't tell gender yet. Anyway, I have had her for about a month and a half, maybe two. To the issue. She is sooo untame! It's very hard to catch her, and when you do, she screams these duck like sounding honks, and will squirm so much it's hard to even hold on. So much so you have to grab her feet. I handle her all the time so I don't know why she still fights me! Also, what if she turns out to be a rooster? Could she be aggressive?
It is likely she/he just does not like being held. People say about their chickens liking to be held, and being so tame, when in reality most chickens do not want to be picked up or fussed with. You can not expect them to like it just because some people's might. It is probably scary for a chick. Their natural mothers would not hold them. The only time they would be grabbed in the wild is when something is trying to eat them.
My orphingtons are 2 of the 3 most standoffish in my flock. The other is my EE hen. They will come around my feet if I am feeding them, but if I try to touch them, they flit away as quickly as they can. I have others that will walk on my feet and stand there to let me pet or pick them up. Not the BO's though! They've been that way since they were chicks and they'll be two in February.
Please tell us how old this Orpington is. Is she the same age as the others? You've had her two months. Have you attempted to sit down in the run and let these new chickens become accustomed to you on their level? It's natural for them to be wary of you if you are standing and walking around and not getting down low so they can really see who you are. It helps to have treat sessions where you can allow them to eat from your hand. After they become comfortable being close to you and eating from your hand, touch them while they take treats from you. It won't take very long for all of them to become more tame than they are now. Chickens are corruptible that way. But there will always be some who really hate being touched. I have a GLW hen who has never warmed up to me in all her six years. In fact, if I try to hold her, she screeches and claws and bites until I release her. She's the only one out of 18 chickens that is completely adverse to being held, and I just have to accept that it's the way she rolls.