Is the chick feed medicated? Some have different opinions on this, but Since they aren't feeling well, my thought might be that they are eating too much medicated food and getting something they really don't need or shouldn't have. Most farm stores have electrolite stuff - kinda like "save-a-chick" that you can add to water. You can also put in a little bit of sugar in the water too to see if they spry up.
What is the temperature of the brooder? Is it too hot for them? My ducks hated anything over 85!! Maybe someone else can help you, these are just my ideas. Good luck!