CSS of child theme sometimes not overriding main

I am working with a child theme of twentythriteen.

For most changes, the child theme style.css is working.

One place that I am really struggling with is the nav bar. Most of the css I enter is just not working, such as getting rid of the italic on focus and changing the color and adding border-radius on hover.

Any ideas of what could be wrong? I thought that the whole style sheet would override the main, but it doesn’t seem to be.