The only time I have had a crop problem was recently in a previously very healthy hen, who suddenly had lost a lot of weight, and she had a squishy slow emptying crop for about a month before she died of what I thought to have been egg yolk peritonitis. It never smelled sour, but each day it blew up with air after she ate her egg. It would go down somewhat by morning. Slow crops can accompany other diseases, kind of a symptom. In coccidiosis, Mareks disease, Eyp, and other it can happen simultaneously. I'm not saying much here I guess, other than to look beyond the crop, because it can be something more.