I'm sorry for your lossShe was always smaller than the other birds and wasn’t growing. But she could eat with the biggest roo. Do you know if marek’s causes this kind of wasting? Or maybe it could just make her more susceptible.

When you lost another recently, did you have confirmation of Marek's through necropsy at your state lab? If not, then consider refrigerating the body of this one and sending it in so you know what you are dealing with.
Marek's can cause wasting, but there's 100s of things that can affect a chicken.
I don't see your location in your profile, so here's a list of state labs.
https://www.metzerfarms.com/PoultryLabs.cfm