9 month old pullet, not eating, listless, pale, Vet doesn't even know whats wrong!

Nine months ago I ordered 4 chicks from MPC, with Mareks vaccines. 3 of the 4 matured nicely and are the picture of health and laying, The 4th (Sofie) has always been a bit timid and for that reason I have paid special attention to her and took to hand feeding her since she's always seem to back away from all the others fighting to get treats, etc when I'd enter the run. She'd hop up into my lap to get her treats. But she never gained the red comb or wattles the others did, and she wasn't laying. ( I know because I have selected my chickens based on egg color produced and she's my Black Copper Marans and supposed to me the dark brown egg) Anyway, I noticed all this and that she didn't seem to be thriving. AND she was getting lighter. So I separated her from the others so she could get a good meal without having to fight for it or wait on me. But this showed she wasn't eating AT ALL on her own so I took her to an avian vet who did a few tests and said looked like no infection, no mites, and Sofie was good enough to provide a healthy looking poop for a fecal test to prove no parasites, but she was way under weight for her breed, anemic and had poor range of motion in one wing and in her beak but could not say what was really wrong. The vet suggested she may have fallen and hurt her wing and jaw and they set badly, or early arthritis (??) and prescribed an anti inflammatory and softened food/vitamins/electrolytes in a dropper till she gained strength. It's been 3 weeks of me dropper feeding this poor thing and though she still eats it is only because I make her..she has no strength. when I put her down she just stands, head and tail down. She may take a few step to her water, and after I leave and come back she may be in a different spot, so she DOES move, but not much. and her breathing seems labored now ( not raggy or wheezing - just I can see her body move up and down as she breaths). She goes limp when I pick her up but stands fairly well when I feed her. This morning it it took longer for her to swallow her food then usual. I think she's giving up. I have no idea what its wrong with her. My DH thinks it's time to put her down. Anyone know what to do??