NEED SOME ADVICE!

I have 6 ducks, one of them is a cayuga duck.. I've been letting them out in the morning this week and in the middle of the week I noticed that her head was underneath her belly, I picked her up and put her on the floor while the rest of my ducks went to their food and water, normally when we go to pick any of them up they flap around as their not use to human handling but she did nothing, just make a low noise in her throat and that was it, when I put her down she still had her head and neck tucked under her belly, she was trying to walk around and was crashing into the fence of the pen etc, I had to stand with her until eventually she lifted her head up as I was worried she may fall down our banking. We've been keeping a close eye on her since then but she still isn't any better, my dad checked her over earlier for mites, fleas, ticks, etc but we cannot see any, he then put her down and she did it again, when I first seen her I thought she'd damaged her neck but we've checked, it feels and looks fine.. it basically looks like she has a broken neck as it goes very limp, we were just wondering whether anyone's had this problem before, what it could be and how to stop it? some people have said it could be the water but they all drink from the same water source so wouldn't all of them be like it? and two people have asked us whether we've wormed them as she could have worms, which we've had them for nearly a year now and haven't wormed them, we also don't think the owner before us wormed them either.
Thanks in advance.

 

Oh god, certainly don't want that to happen.. it's worse as she's had it for a few days but thanks for the advice!