All of my custom Bootstrap styles are not working in my child theme!

In your child theme’s function.php file, find the enqueue function and add it there instead of using @import in your CSS.

For example:

function my_scripts_and_styles() {
    wp_enqueue_style( 'main-style', get_stylesheet_uri() );
    wp_enqueue_style( 'bootstrap-style', get_template_directory_uri() . '/css/bootstrap.min.css', array( 'main-style', '', true );

    if ( is_singular() && comments_open() && get_option( 'thread_comments' ) ) {
        wp_enqueue_script( 'comment-reply' );
    }
}
add_action( 'wp_enqueue_scripts', 'my_scripts_and_styles' );

What this function will do is load your style.css file, then load bootstrap.min.css only if style.css is successfully loaded (NOTE: It assumes bootstrap.min.css is located in /wp-content/themes/{your theme}/css so adjust that code accordingly.

wp_head() will take it from there. If you need bootstrap to load first, then move the array() argument to main-style and change the contents to 'bootstrap-style'.