That's really an amazing story! Perhaps she didn't get the memo that she's not a Call Duck!
Yes, nutritional issues (from before hatch) sometimes take a while to catch the baby up - so I'm glad she's getting her feathers.
Once in a while, batches of babies will have one or two that are dinky, or different, or slow. It happens more often the less new blood brought in.
I had one like that which I loved dearly. "Wacky" because she was pretty wacky. She started off life hard as she hatched late, by a day, in the 100 degree heat we had. When I found her, she was sucked into a mud puddle and covered with fireants. Only her head was showing.
When I cleaned her off, I knew immediately she wasn't a normal duckling. And after 2 days of eating, she would move like greased lightening - zipping everywhere in her little cage. (Thus her name). She grew up to be a very special girl. She's go to the front door and quack for me to open it. If I wouldn't, she'd go to the back door and do the same. Eventually I'd open the door and, of course, I'd have to put her in the feed bin and let her eat secretly - how could I say "no" to a duck that quacked at both doors?