I'm sorry for your loss.
Sometimes it can be just a case of "failure to thrive" or some type of genetic/developmental defect.
If you are having trouble with chicks from the flea market or auction you may want to ask the seller more questions such as vaccination details, bio-security, breeding program, etc.
You may also want to consider what new chicks are being exposed to if you aren't cleaning/sanitizing your brooder between new batches of chicks. Dust, dander etc., can hold and "pass along" some illnesses that young ones may not have a strong enough system to overcome.
Also you may want to keep a record of when, where and who you got chicks from, what symptoms/signs the chicks presented, that way you can start narrowing down (hopefully) the cause of loss and make changes to prevent it.