I still don't see an issue with head shaking?
Maybe
@coach723 or
@Eggcessive can spy something?
I don't see a problem with you giving the Infant Vitamin 2-3 drops twice a day for 3-5 days - you can overdo vitamins.
You mention Ivermectin! Are you seeing lice or mites on the chicks? I would
not use Ivermectin on chicks at all. If you have lice or mites then use a Permethrin based powder to dust the birds.
They are getting fairly large, can you provide them with a bit more space?
Chicken feed - is it starter that you are giving? I would offer it free choice, let them eat what they want of the feed during waking hours.
Again - where are you located? Are you in the US or another country?