It sounds like it may be a pendulous crop rather than a sour crop. If it's sour, you will smell it on her breath and she will be reluctant to eat. A pendulous crop can develop as a result of a partial impaction and can increase the likelihood of an impacted crop. The muscle of the crop becomes stretched and less elastic and sometimes a pocket forms below the outlet to the digestive tract and silt builds up in the pocket and makes it sag further. I've managed to clear a partial impaction by regular massage and sloppy feeds only with no access to fibrous material like bedding. It took almost two weeks and my hen was skin and bone under all those feathers because she was starving. I've also had one that I had to do crop surgery on to remove the debris.
How does your bird's keel bone feel.... reasonably well padded or rather sharp with very little/no muscle covering, suggesting that she is not getting enough food through her system to maintain body weight. Compare to other normal birds of the same age. Once you are sure that the debris in her crop has been broken up by massage and passed down into her digestive tract (or vomited up which needs to be done very carefully to avoid aspiration of the vomit into the respiratory tract) a crop bra may help prevent sediment/debris building up in her crop again. If you cannot shift the mass after prolonged treatment, and she is losing weight then you may have to consider crop surgery. Hopefully it won't come to that, but it is important to monitor her body condition because those feathers can hide an emaciated chicken beneath them.
Good luck with her
Barbara