Either or . . . brewer's yeast or niacin in water.
Keep it up till the ducklings are at least ten weeks old, longer if there still seems to be some nervous system trouble.
And keep us posted!
If you gave the wheat to all of them and only one is having trouble, it is not likely the problem. But it appears that some ducks are more sensitive to vitamin deficiencies or toxins than others.
Go with the niacin, and poultry vitamins.