A video of the sick ones may be helpful, you can post it here or give a link to youtube - not everyone has facebook so if it's posted there, you will limit viewing.
Can you separate the sick ones so you can monitor their food/water intake and their poop?
If you have a vet they can do a fecal...
Welcome to BYC
What type of foods/treats are you feeding them?
Do they have access to grit?
It almost sounds like they are aspirating on their vomit.
Are the crops not emptying at all? Full crop at night - empty crop by morning?
What does the poop look like?
Any smell to the liquid in that...