The duckling your talking about @Gallinarium died 

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Sure I will try to get all the specifics but I must say this. We have some other ducklings from tractor supply that are more than a month old. They used the exact same wood shaving, the exact same box, the same heater and everything. The only difference is the TS ducklings had different food than these ones for their first few weeks.@Crazy4Fowl can you explain what your brooder is what your using for bedding what the temps are and what feed are you using?

I got 15 chicks from a hatchery, they are not even a week old, and two of them have died already. They looked a lot like your duck, as they were laying down and moving their heads weird. We haven't figured out for sure what it is, but I think wry neck.
Wry neck can be caused by a vitamin E deficiency, or a selenium deficiency.
Try to get them supplements as fast as you can.
Thank you. Got them wednesday and they took two days to ship so I would say they are almost a week old.Sorry for your losses. How old are they?