Post Header Full-Width

I’m trying to get my Post header to go full-width. It used to be full-width, so I don’t know if it’s WP upgrade that has altered it. Or something else?
I’ve tried the following four bits of code:

.post-header-image img {
Margin: auto;
}

and

.post-header-image img {
    width: 100%;
}

and

.entry-header img.wp-post-image {
   width: 100%;
}

and

header#masthead {
    background-size: cover;
    background-repeat: no-repeat;
}

Has anyone got any idea why none of these are working?
Thanks for any suggestions

The page I need help with: [log in to see the link]