I am having a very similar problem with one of my new chicks. She is a Buff Orphington and the first few days home from the feed store, she was running around and peeping up a storm. Then she started just staying down on her hocks, sleeping and when she was awake was losing her balance while standing up to groom or being knocked over by another chick. I thought for sure, I would have to have her euthanized but wanted to really give her a chance to get over whatever it was.
I started reading about the Poly Sol vitamins ( no iron) and I have definitely seen a big improvement in her ability to stand, walk, keep balance and even knock over the other chick for food! She still does go down on her hocks but instead of staying down for long bouts of time, she is down for a minture or two then pops back up and trots all over the brooder. She does still poop out and need to rest, but so does the other chick so I'll assume they're just doing what chicks do.
I started giving her greek yogurt with a swirl of the vitamins and putting drops of vitamins in their quart size waterer a few days ago. Still keeping up the regimen and will do it while in their chick hood. I also purchased Raw Unpasturized Apple Cider Vinegar and plan to add it to their diet, but when they cycle down from their vitamins.
I really don't want them off their food completely since eating spiked yogurt makes them wipe their beaks on the paper towels! (but they keep coming back for more!!)
To make a long story short...visit your local grocery/drug store and pick up the Poly Vi Sol (no iron) liquid vitamins and add it to their food and water. I don't think it can really hurt (unless their is a major problem) .... and search for more information on the board..