I have two ducklings that were hatched on August 11 so they are just over two weeks old. One has not grown a spec since I got them last week. The other has doubled in size. The small one also flipps over constantly and can't get up. She eats and drinks normally and is pooping fine. She seems to have a small growth on the side of her head but besides that seems healthy except the constant flipping and small size. These are my first ducklings as my other ducks were obtained as adults. Is this something that she'll grow out of or is it a matter of time before she dies? Thanks for any help.
		
		
	 
This sounds like B vitamin deficiency - particularly niacin.  Here is my writeup.  Please get them both on supplements.  What do they get for food?
 
Here is my writeup on B vitamin supplements.
 
 
You have some options.
 
I would go with brewer's yeast.  Forgive me for using capital letters, but for anyone else who may casually look over this thread, NOT baking yeast, NOT winemaking yeast.  Brewer's yeast is a nutritional supplement and it provides niacin and a few other vitamins.
 
Generally, folks who supplement with brewer’s yeast use a tablespoon per cup of food.
 
Ducklings need about three times the niacin chicks do.  And some ducklings get leg problems or seizures if they don't have enough niacin.
 
If you cannot find the brewer's yeast, or if you happen to have (here come the capital letters again, please forgive me) PLAIN niacin - NOT no-flush, NOT timed release, then dissolve 150 mg niacin per gallon of water for the duckling’s drinking water.
 
Another approach is to use B Complex capsules, and use the B3 - niacin - levels as a guide.  Again,  150 mg B3 per gallon of drinking water.
 
 
Keep the little one on the supplements for about 8 weeks.  
 
Sometimes it only takes a few days to see improvement.
 
You can also see how it went for Qwackers and Bentley from these threads.
 
https://www.backyardchickens.com/t/857143/treating-wry-neck-in-crested-duckling-edited-title
 
https://www.backyardchickens.com/t/761308/peking-duckling-with-crooked-neck-help
 
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Have you read the Raising Ducklings sticky?
 
https://www.backyardchickens.com/t/750869/raising-and-caring-for-ducklings#post_10611711