could she have worms? not usually my first thought, but since she is so thin, tough to worm a sick/weak chicken though. But may be the problem too. try giving her some electrolyte solution in her water and some chicken vitamins to help give her some strenght. I have read that mites can cause a hen to get weak, they suck their blood out at night, have you checked for bugs? sounds like you are doing all you can for the possible crop issue, keeping her in will help her out. hope she does well, good luck