Some chick vitamins do not contain riboflavin, so check the label. Human B complex 1/4 tablet daily can be crushed in a spoonful of water and fed, or put in a bit of egg.
I would give vitamins with riboflavin or B 2. Dip its beak often in the water several times to get it drinking. Make a little watery chick crumble and water mash, and offer that. Has there been any pasty butt? What temperature is your brooder?