Grit. I think you have found at least part of the problem. The potential problem with "natural" grit that is found in dirt is that the edges might be rounded from wear. Granite grit has nice jagged edges. Can you get some? If her crop is painful, this concerns me. Is an avian vet an option? Impacted crop is usually treated with gentle massage, but if this causes pain, it's possible she will need it emptied surgically. I do think it could be the sunflower shells if there isn't good grit available. Do they have access to grass or anything else besides the feed and treats you give them? Good luck. Others with more experience than I have have already been tagged. There are others, as well, but this is a good group. I hope we can help you help your girl.