Recently I was working with an issue in WordPress where the site’s menu was not showing up on an archive page for one of my custom post types. There were a few troubleshooting things I tried, including the most common recommendation, use theme_location instead of menu when referencing the menu in your theme (code after the jump):

wp_nav_menu( array(
    'theme_location' => 'main-menu-slug',
) );

However, that wasn’t working. I did find another solution that got me part of the way there, which removed the taxonomy specifications if WordPress was building the menu.

function fix_nav_menu( $query ) {
    if ( $query->get( 'post_type' ) === 'nav_menu_item' ) {
    $query->set( 'tax_query', '' );
    }
}
add_action( 'pre_get_posts', 'fix_nav_menu' );

This didn’t completely work for me because I was using a meta_key to sort my posts, so I changed the if statement to reset a couple more query variables:

if ( $query->get( 'post_type' ) === 'nav_menu_item' ) {
    $query->set( 'tax_query', '' );
    $query->set( 'meta_key', '' );
    $query->set( 'orderby', '' );
}

That did the trick! It took me quite a bit of troubleshooting, so hopefully this will help you out if you’re trying to fix wp_nav_menu on Custom Post Type Archives.