can be a lot of things unfortunately. are her legs splayed out? if so you may want to make a splint to bring them together in a more natural stance. may be unrelated but i noticed it is crested. the crest can lead to several "defects" and leg weakness is one of them. go ahead and start providing it with some niacin. you can use brewers yeast if you can't find just niacin. just make sure to stay away from flush free and slow release. the niacin is good for all ducks and aids in their development. crested issues affected one of ours and she still wobbles and stumbles around but is very independent. just a thought anyways, maybe someone with more experience can give better info.