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?