Crop problems can be simple or complicated. I'm going to give you a link to a pretty comprehensive article that should help you at least narrow it down. If you go through the info step by step, hopefully it will help point you in the right direction.
Also, oyster shell is a soluble source of calcium, it is not the equivalent of grit. Grit is a non soluble material, usually granite, that is used in the gizzard to grind the food. They serve very different purposes and are not interchangeable, both should be available to your birds free choice all the time, they will take what they need. Sour crop usually causes a bad odor from the chickens mouth. The previous post suggestion would be a good starting point, to see if the crop goes down or not. If the crop is full at bed time (remove feed) and empty 1st thing in the morning before feeding, then it's probably ok and as said, a big eater. Crop fullness will vary throughout the day as they eat and digest. If the crop is not emptying, then you have something going on.
https://www.backyardchickens.com/ar...d-sour-crops-prevention-and-treatments.67194/