The problem is that WordPress load always first the style.css file of your child-theme, then all other stylesheet files and then, after all, the original style.css file of the parent theme. This will always overwrite the changes you made in your child-theme and all stylesheet rules of the other files. I’m sorry, but that’s the way WordPress set up the child themes. Just check Google, all other have the same problem. For this I told you that it’s better to use child-themes only if you know what you do, because you have to sort the different stylesheet files manually.