It's funny that you are having this issue with the Copper Marans.  I got 2 of them last year.  Both are gone now.  One had serious sour crop/impaction issues for life until she finally died of them.  I never figured out what actually was impacting her, despite much effort, surgery and eventual necropsy.  I did use the Monistat on her with good effect, though, in the midst of our unlucky adventures.  I got rid of the sour crop only to have her actually succumb to an impaction lower down in the GI system.  I am still thinking that there may have been some GI motility issue going on with this bird from day one.
 
The other chick had blown air sacs from the day I got her.  She also died very young, but a lot less dramatically.  I never expected her to live, though, so it came as no great surprise when she just dropped dead one day.
 
I am wondering where you got your chicks from.  Do you mind if I ask?