Header Image won't stretch to fill screen width

I have a brand new project I’m working on. I’m using twentyfourteen for the first time. I’m developing on a desktop with 1260X800 resolution and it seemed okay at first. Had a problem with it not filling the screen on my laptop which is 1440×900. I fixed that by adjusting the max-width for both the .site and .header parameters in my syles.css in the child theme. I thought that had fixed it but I’m finding that the header image still won’t stretch to meet the wider (1440) screen. The image size is 1440×288 so it should be fine. Am I missing a setting somewhere?