Help!

I have 27 chickens about 3 weeks old. Two days ago some of them started acting like they couldn't walk. Like they had weak legs, couldn't support their weight. Two are to the point of laying on their sides all day, not being able to get up to eat or drink (I have to help them). It's like they can't keep their legs under them!

The others, that aren't quite as bad, stumble when they walk, are unsteady, easily fall over, prefer laying down. No other symptoms that I can see.

They eat a chick starter food, have water with added electrolytes and vitamins. We clean their cages once a day and change their water at the same time.

What am I missing?

This is my third year raising chicks and I've never had this problem before...

Sure appreciate any help or insight you all can give me!