Sorry, I meant a deformity (Not external, maybe internal) in the chick. You did mention that you've hatched from the same birds and had no problems like this, so I guess deformity may not be a valid point. Maybe, maybe not. I do think it is some sort of deficiency in nutrients though. What are...
Hmmm... my only other guess is a disease among your chicks as my quail died the same exact way (Same symptoms too). I pretty sure they are too weak and are not getting enough nutrients or something along that line. Maybe @Nabiki could help.
If your chicks are falling on their backs, then there is a serious problem. I've had several of my birds die that way (They can't breathe well in that position). Also added with panting, these symptoms make me think that your chicks do not have adequate ventilation. Is the brooder airy and...