Bald wings on duckling

[Quack and Waddle]

My 3 week old duckling who currently lives in the house is gradually losing her down, but much slower than her 4 week old siblings who are outside. Shes the one we had to rescue from her shell, and while she is very active and eating, she is still very small.
What worries me is the tops of her wings are becoming bald. I’m wondering if she’s over grooming or feather plucking because of loneliness or boredom. I’m hoping that’s it’s not some sort of health problem. She eats Purina crumbles, mealworms and chopped lettuce, as do the others.
Anyone have ideas? Thank you!!!!

 

[CherishedFowlGuardian]

Is she the only duck inside, and does she preen a lot?