Strange Behavior from 2 week old duck

[ChickMomma]

I have 4 peking ducklings that are 2 weeks old. Three are huge (maybe 4 times the size from when I got them) but one is half the size of the others. She (I am hoping) is keeping her head tucked back into her body and is not eating and drinking as vigorously as the other three. She also does not look up and peep when they are startled to see what the noise is. They are being fed chick starter / grower and I put vitamin / electrolyte stuff in their waterer about 3 times a week. They have been outside one day for several hours when it was 75+ degrees and sunny. I also feed them peas as a treat with grit sprinkled on top!
Any suggestions as to what could be wrong?
Thanks
Susanne

 

[raindrop]

Are you supplementing the chick starter with niacin? Chick starter is deficient in niacin. I would switch to Purina Flock Raiser or a gamebird starter.