How is the temperature where they are housed? Is it too hot? At a couple of weeks old the temp should be about 90 degrees. You drop it 5 degrees for each week thereafter until fully feathered and no heat is needed, usually between six and eight weeks.
Adding other chicks from another source could have brought something over also.
Also diets deficient in certain vitamins/minerals can cause a host of growth problem and health problems. That includes a chick hatched from a hen deficient in such. I found many feeds sold in feed stores are old and have been stored longer than the normal three months. I was told by a couple of manufacturers that feed should ideally be used within three months from date of manufacture. Those with high moisture content break down quickly after three months. They can store a little longer in cooler weather. I've found feeds six months and some even a year old being sold in stores. So, I have started adding a good broad range vit/mineral supplement to my birds water since most of the feed I find around here seems to be right at the expiration date.
Here's a good link to see what problems can be caused by a mineral or vitamin deficiency:
http://www.merckvetmanual.com/mvm/p..._poultry/mineral_deficiencies_in_poultry.html
Vitamin destruction in feeds is a factor of time, temperature, and humidity. For most feeds, efficacy of vitamins is little affected over 2-mo storage within mixed feed.
http://www.merckvetmanual.com/mvm/p..._poultry/vitamin_deficiencies_in_poultry.html
I have issues around here with time of manufacture (old feed in stores) and heat and humidity were definitely factors during the spring and summer months.
