Is it possible that she's had her feathers pulled by one of the drakes? I have a female who pulls feathers on my other females and before I realized that I spent hundreds at the vet trying to figure out what was going on. Attached is a photo of my duck with feathers pulled where I thought she had been wounded somehow. Just a thought. I hope your duck finds a happy home with some other females! 
		
		
	
	
		
	 
		
	
	
		
			
		
		
	
				
			
	