Warning: super gigantic post!
Sometimes a vitamin deficiency will contribute to stunted growth,
@Isaac 0 would definitely be the person to talk to If it's a deficiency. I'm kinda thinking it's vitamins, if she's guzzling up food but not growing then it would seem like she is trying to get some kind of missing nutrient but not being satisfied. It's kinda like when our bodies are telling us that we are lacking in certain vitamins and we automatically assume we're just hungry and drive through mcdonald's and get a coke and a burger, not really getting what we need, just masking the feeling. The only other explaination would be that she can't properly digest her food and thats why she's not growing. Maybe it's also a missing amino acid in her diet? Amino acids are like the building blocks for our bodies, e.g. pigs cannot convert feed into muscle mass without lysine, lysine is found in cow poop so lots of farmers will cover cow poop ( cows also don't completely digest their food) with mollasses and feed and voila! Lysine for the pigs! Maybe you could add in some greens? I haven't done research on what kind of amino acids ducklings need so maybe someone more knowledgeable could comment.
Lots of people ferment their duck feed because it makes the nutrients more available, but I'm not sure about fermented feeds for ducklings. You're doing good by giving her eggs. It's strange that the others are growing but she isn't, however if it's a deficiency or something a little more genetic then sometimes it won't affect her siblings or brood mates and she will be the only one to struggle. Hope this helps, it was basically just me trying to figure out why ducklings won't grow
She’s eating like crazy! I dunno where it’s going 
