wp_enqueue_scripts hook is not being called

I added the following code to my functions.php:

if(!function_exists('bi_frontend_scripts')) {
    function bi_frontend_scripts() {
        wp_enqueue_script('jquery');
        // I originally wanted to do:
        wp_enqueue_script('jQuery.bxSlider', get_bloginfo('template_url').'/scripts/jquery.bxslider/jquery.bxslider.min.js', array('jquery'));
    }
}
add_action('wp_enqueue_scripts', 'bi_frontend_scripts');

But apparently neither of the scripts is being enqueued. I didn’t get what my problem was, so I added this script to find out if the hook is called:

function aal_handler() {
    static $list = array();
    // exclude certain actions:
    $exclude = array('gettext', 'gettext_with_context');

    $action = current_filter();

    if(!in_array($action, $exclude)) {
        $list[] = $action;
    }
    // shutdown is the last action
    if ('shutdown' == $action) {
        print '<pre>' . implode( "\n", $list ) . '</pre>';
    }
}
add_action('all', 'aal_handler', 99999, 99);

And in the list wp_enqueue_scripts does not appear. So why the hook is not called?!

Your code is correct, though I would not add the callback if the function name has already been used. If someone else has used the name you don’t know what you might be adding.

if(!function_exists('bi_frontend_scripts')) {
    function bi_frontend_scripts() {
        wp_enqueue_script('jquery');
        // I originally wanted to do:
        wp_enqueue_script('jQuery.bxSlider', get_bloginfo('template_url').'/scripts/jquery.bxslider/jquery.bxslider.min.js', array('jquery'));
    }
    add_action('wp_enqueue_scripts', 'bi_frontend_scripts');
}

I also tested your “hook dump” function– aal_handler()— and that works. wp_enqueue_scripts is part of the output.

I have to conclude that there is something wrong with the theme. wp_enqueue_scripts is hooked to wp_head, which fires in the wp_head() function. The most obvious possibility is that your theme is not using wp_head() correctly, or at all. Another distant possibility is that something has removed your callback– for example, with:

remove_all_filters('wp_enqueue_scripts');