I've noticed that very large photos eg 3000x2000, just seem to make the crop loader spin without ever finishing.
I've added a plugin that changes the fullsize image to be a lot more manageable but found that certain portrait images just won't crop.
The image i am using has been resized to be 1600x2400 upon upload. Other images that are landscape eg. 1600x1020 crop fine.
Could it be a memory issue on the server? I never get an error, just the crop loader never finishes.